client API: expose OpenGL renderer

This adds API to libmpv that lets host applications use the mpv opengl
renderer. This is a more flexible (and possibly more portable) option to
foreign window embedding (via --wid).

This assumes that methods like context sharing and multithreaded OpenGL
rendering are infeasible, and that a way is needed to integrate it with
an application that uses a single thread to render everything.

Add an example that does this with QtQuick/qml. The example is
relatively lazy, but still shows how relatively simple the integration
is. The FBO indirection could probably be avoided, but would require
more work (and would probably lead to worse QtQuick integration, because
it would have to ignore transformations like rotation).

Because this makes mpv directly use the host application's OpenGL
context, there is no platform specific code involved in mpv, except
for hw decoding interop.

main.qml is derived from some Qt example.

The following things are still missing:
- a way to do better video timing
- expose GL renderer options, allow changing them at runtime
- support for color equalizer controls
- support for screenshots

DOCS/client_api_examples/qml/main.cpp

@@ -0,0 +1,19 @@
+#include <QGuiApplication>
+
+#include <QtQuick/QQuickView>
+
+#include "mpvrenderer.h"
+
+int main(int argc, char **argv)
+{
+    QGuiApplication app(argc, argv);
+
+    qmlRegisterType<MpvObject>("mpvtest", 1, 0, "MpvObject");
+
+    QQuickView view;
+    view.setResizeMode(QQuickView::SizeRootObjectToView);
+    view.setSource(QUrl("qrc:///mpvtest/main.qml"));
+    view.show();
+
+    return app.exec();
+}

DOCS/client_api_examples/qml/main.qml

@@ -0,0 +1,39 @@
+import QtQuick 2.0
+import QtQuick.Controls 1.0
+
+import mpvtest 1.0
+
+Item {
+    width: 1280
+    height: 720
+
+    MpvObject {
+        id: renderer
+        anchors.fill: parent
+
+        MouseArea {
+            anchors.fill: parent
+            onClicked: renderer.loadfile("../../../test.mkv")
+        }
+    }
+
+    Rectangle {
+        id: labelFrame
+        anchors.margins: -50
+        radius: 5
+        color: "white"
+        border.color: "black"
+        opacity: 0.8
+        anchors.fill: label
+    }
+
+    Text {
+        id: label
+        anchors.bottom: renderer.bottom
+        anchors.left: renderer.left
+        anchors.right: renderer.right
+        anchors.margins: 100
+        wrapMode: Text.WordWrap
+        text: "QtQuick and mpv are both rendering stuff."
+    }
+}

DOCS/client_api_examples/qml/mpvrenderer.cpp

@@ -0,0 +1,125 @@
+#include "mpvrenderer.h"
+
+#include <QObject>
+#include <QtGlobal>
+#include <QOpenGLContext>
+
+#include <QtGui/QOpenGLFramebufferObject>
+
+#include <QtQuick/QQuickWindow>
+#include <qsgsimpletexturenode.h>
+
+class MpvRenderer : public QQuickFramebufferObject::Renderer
+{
+    static void *get_proc_address(void *ctx, const char *name) {
+        (void)ctx;
+        QOpenGLContext *glctx = QOpenGLContext::currentContext();
+        if (!glctx)
+            return NULL;
+        return (void *)glctx->getProcAddress(QByteArray(name));
+    }
+
+    mpv_opengl_cb_context *mpv_gl;
+    QQuickWindow *window;
+public:
+    MpvRenderer(mpv_opengl_cb_context *a_mpv_gl)
+        : mpv_gl(a_mpv_gl), window(NULL)
+    {
+        int r = mpv_opengl_cb_init_gl(mpv_gl, NULL, get_proc_address, NULL);
+        if (r < 0)
+            throw "could not initialize OpenGL";
+    }
+
+    virtual ~MpvRenderer()
+    {
+        mpv_opengl_cb_uninit_gl(mpv_gl);
+    }
+
+    void render()
+    {
+        assert(window); // must have been set by synchronize()
+
+        QOpenGLFramebufferObject *fbo = framebufferObject();
+        int vp[4] = {0, 0, fbo->width(), fbo->height()};
+        window->resetOpenGLState();
+        mpv_opengl_cb_render(mpv_gl, fbo->handle(), vp);
+        window->resetOpenGLState();
+    }
+
+    QOpenGLFramebufferObject *createFramebufferObject(const QSize &size)
+    {
+        QOpenGLFramebufferObjectFormat format;
+        format.setSamples(4);
+        return new QOpenGLFramebufferObject(size, format);
+    }
+
+protected:
+    virtual void synchronize(QQuickFramebufferObject *item)
+    {
+        window = item->window();
+    }
+};
+
+MpvObject::MpvObject(QQuickItem * parent)
+    : QQuickFramebufferObject(parent)
+{
+    mpv = mpv_create();
+    if (!mpv)
+        throw "could not create mpv context";
+
+    mpv_set_option_string(mpv, "terminal", "yes");
+    mpv_set_option_string(mpv, "msg-level", "all=v");
+
+    if (mpv_initialize(mpv) < 0) {
+        mpv_terminate_destroy(mpv);
+        throw "could not initialize mpv context";
+    }
+
+    // Make use of the MPV_SUB_API_OPENGL_CB API.
+    mpv::qt::set_option_variant(mpv, "vo", "opengl-cb");
+
+    // Request hw decoding, just for testing.
+    mpv::qt::set_option_variant(mpv, "hwdec", "auto");
+
+    mpv_gl = (mpv_opengl_cb_context *)mpv_get_sub_api(mpv, MPV_SUB_API_OPENGL_CB);
+    if (!mpv_gl) {
+        mpv_terminate_destroy(mpv);
+        throw "OpenGL not compiled in";
+    }
+
+    mpv_opengl_cb_set_update_callback(mpv_gl, on_update, (void *)this);
+
+    connect(this, &MpvObject::onUpdate, this, &MpvObject::doUpdate, Qt::QueuedConnection);
+}
+
+MpvObject::~MpvObject()
+{
+    mpv_terminate_destroy(mpv);
+}
+
+void MpvObject::on_update(void *ctx)
+{
+    MpvObject *self = (MpvObject *)ctx;
+    emit self->onUpdate();
+}
+
+// connected to onUpdate(); signal makes sure it runs on the GUI thread
+void MpvObject::doUpdate()
+{
+    update();
+}
+
+void MpvObject::loadfile(const QString& filename)
+{
+    QVariantList cmd;
+    cmd.append("loadfile");
+    cmd.append(filename);
+    mpv::qt::command_variant(mpv, cmd);
+}
+
+QQuickFramebufferObject::Renderer *MpvObject::createRenderer() const
+{
+    window()->setPersistentOpenGLContext(true);
+    window()->setPersistentSceneGraph(true);
+    return new MpvRenderer(mpv_gl);
+}

DOCS/client_api_examples/qml/mpvrenderer.h

@@ -0,0 +1,33 @@
+#ifndef MPVRENDERER_H_
+#define MPVRENDERER_H_
+
+#include <assert.h>
+
+#include <QtQuick/QQuickFramebufferObject>
+
+#include "libmpv/client.h"
+#include "libmpv/opengl_cb.h"
+#include "libmpv/qthelper.hpp"
+
+class MpvObject : public QQuickFramebufferObject
+{
+    Q_OBJECT
+
+    mpv_handle *mpv;
+    mpv_opengl_cb_context *mpv_gl;
+
+public:
+    MpvObject(QQuickItem * parent = 0);
+    virtual ~MpvObject();
+    Renderer *createRenderer() const;
+public slots:
+    void loadfile(const QString& filename);
+signals:
+    void onUpdate();
+private slots:
+    void doUpdate();
+private:
+    static void on_update(void *ctx);
+};
+
+#endif

DOCS/client_api_examples/qml/mpvtest.pro

@@ -0,0 +1,11 @@
+QT += qml quick
+
+HEADERS += mpvrenderer.h
+SOURCES += mpvrenderer.cpp main.cpp
+
+CONFIG += link_pkgconfig debug
+PKGCONFIG = mpv
+
+RESOURCES += mpvtest.qrc
+
+OTHER_FILES += main.qml

DOCS/client_api_examples/qml/mpvtest.qrc

@@ -0,0 +1,5 @@
+<RCC>
+    <qresource prefix="/mpvtest">
+        <file>main.qml</file>
+    </qresource>
+</RCC>