* include/video.h, include/vlc_config.h, src/video_output/*: changed the
picture buffer allocation scheme to allocate pictures from the render
heap in a clockwise fashion instead of always picking the first available
one from the start of the heap.
This allows us to benefit from ffmpeg's macro-block skipping feature.
As a side effect, we also have less chance of reusing a refrence picture
buffer when we can't link/unlink them.
* modules/codec/ffmpeg/video.c: modified the direct rendering code to benefit
from the macro-block skipping algorithm.
A new flag has also been introduced in the video_output's render heap to
forbid modifications to the render picture buffers (for instance to overlay
subtitles).