mp_image: add helper for copying image attributes

This enw function is similar to mp_image_set_params(), but doesn't force
setting "hard" parameters like image size and format.
vo_opengl: add infrastructure for hardware decoding OpenGL interop

Most hardware decoding APIs provide some OpenGL interop. This allows
using vo_opengl, without having to read the video data back from GPU.

This requires adding a backend for each hardware decoding API. (Each
backend is an entry in gl_hwdec_vaglx[].) The backends expose video data
as a set of OpenGL textures.

Add infrastructure to support this. The next commit will add support for
VA-API.
vo_opengl: add support for VA-API OpenGL interop

VA-API's OpenGL/GLX interop is pretty bad and perhaps slow (renders a
X11 pixmap into a FBO, and has to go over X11, probably involves one or
more copies), and this code serves more as an example, rather than for
serious use. On the other hand, this might be work much better than
vo_vaapi, even if slightly slower.
Fix -Wshadow warning about seek function in playloop.c

mpvcore/player/playloop.c: In function 'seek':
mpvcore/player/playloop.c:209:54: warning: declaration of 'seek' shadows a global declaration [-Wshadow]
mpvcore/player/playloop.c:209:12: warning: shadowed declaration is here [-Wshadow]
mpvcore/player/playloop.c: In function 'queue_seek':
mpvcore/player/playloop.c:360:25: warning: declaration of 'seek' shadows a global declaration [-Wshadow]
mpvcore/player/playloop.c:209:12: warning: shadowed declaration is here [-Wshadow]

Signed-off-by: wm4 <wm4@nowhere>