Drop support for old ffmpeg header layout.

xbmc requires a ffmpeg version that has the new layout.
Drop PIX_FMT_VDPAU_MPEG4 checks and assume it is always present. It is the case for all ffmpeg versions xbmc can build against.
configure: remove a no longer valid comment
Restore code parts from [573b4bc18] to improve libav compatibility.
configure: Drop checks for sample_aspect_ratio. They are useless because they are done via #ifery now.