sub, vo_opengl: use packed sub-bitmaps directly if available

The previous few commits changed sd_lavc.c's output to packed RGB sub-
images. In particular, this means all sub-bitmaps are part of a larger,
single bitmap. Change the vo_opengl OSD code such that it can make use
of this, and upload the pre-packed image, instead of packing and copying
them again.

This complicates the upload code a bit (4 code paths due to messy PBO
handling). The plan is to make sub-bitmaps always packed, but some more
work is required to reach this point. The plan is to pack libass images
as well. Since this implies a copy, this will make it easy to refcount
the result.

(This is all targeted towards vo_opengl. Other VOs, vo_xv, vo_x11, and
vo_wayland in particular, will become less efficient. Although at least
vo_vdpau and vo_direct3d could be switched to the new method as well.)

sub/osd.h

@@ -42,6 +42,11 @@ struct sub_bitmap {
     int x, y;
     int dw, dh;
 
+    // If the containing struct sub_bitmaps has the packed field set, then this
+    // is the position within the source. (Strictly speaking this is redundant
+    // with the bitmap pointer.)
+    int src_x, src_y;
+
     struct {
         uint32_t color;
     } libass;
@@ -60,6 +65,17 @@ struct sub_bitmaps {
     struct sub_bitmap *parts;
     int num_parts;
 
+    // Packed representation of the bitmap data. If non-NULL, then the
+    // parts[].bitmap pointer points into the image data here (and stride will
+    // correspond to packed->stride[0]).
+    //  SUBBITMAP_RGBA: IMGFMT_BGRA (exact match)
+    // Other formats have this set to NULL.
+    struct mp_image *packed;
+
+    // Bounding box for the packed image. All parts will be within the bounding
+    // box. (The origin of the box is at (0,0).)
+    int packed_w, packed_h;
+
     int change_id;  // Incremented on each change
 };