* 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).