I choosed to extract the decoded frame back to memory instead of developping
a new specific vaapi vout. It was easier, allows transcoding and does not break
our current OSD.

 Becarefull, you probably have to applied a pending patch on ffmpeg for h264
decoding.

 You still have to manually uncomment the HAVE_AVCODEC_VAAPI define in
avcodec.h and to fix the build system to test it.