The acceleration plugin now returns the expected pixel format instead
of checking it. That way, the avcodec module can check it directly.

This works as long as each acceleration plugin supports only one pixel
format (otherwise a table would be required).
(cherry picked from commit b667100bcf0cbd916db4fbb6ce6f8a51ff971ad8)

Conflicts:
	modules/codec/avcodec/dxva2.c
	modules/codec/avcodec/va.h
	modules/codec/avcodec/vaapi.c
	modules/codec/avcodec/vda.c
	modules/codec/avcodec/video.c