video: move struct mp_hwdec_info into its own header file

This means most code accessing this struct must now include hwdec.h
instead of dec_video.h. I just put it into dec_video.h at first because
I thought a separate file would be a waste, but it's more proper to do
it this way, as there are too many files which include dec_video.h only
to get the mp_hwdec_info definition.

video/hwdec.h

@@ -0,0 +1,20 @@
+#ifndef MP_HWDEC_H_
+#define MP_HWDEC_H_
+
+// Used to communicate hardware decoder API handles from VO to video decoder.
+// The VO can set the context pointer for supported APIs.
+struct mp_hwdec_info {
+    struct mp_vdpau_ctx *vdpau_ctx;
+    struct mp_vaapi_ctx *vaapi_ctx;
+    // Can be used to lazily load a requested API.
+    // api_name is e.g. "vdpau" (like the fields above, without "_ctx")
+    // Can be NULL, is idempotent, caller checks _ctx fields for success/access.
+    void (*load_api)(struct mp_hwdec_info *info, const char *api_name);
+    void *load_api_ctx;
+};
+
+// Trivial helper to call info->load_api().
+// Implemented in vd_lavc.c.
+void hwdec_request_api(struct mp_hwdec_info *info, const char *api_name);
+
+#endif