Add limited support for 4k h264 decoding

This allows decoding up to 3840x2160 frames, but it is too slow
to be useful for watching most 4k videos.
Further disp isn't able to scale down such big pictures properly
at the moment. It could even be a hardware limitation. The Video
Engine can do prescaling while decoding, but VDPAU doesn't provide
mechanisms to properly handle that, as they strictly isolate
decoding and display functions.