VDPAU on AMD

@fritsch and I have worked with the open source devs of AMD who have been implementing vdpau for Radeon. After more than 2 years of pain with XvBA AMD hardware for Linux get attractive again. @Anssi has patched ALSA and made 8 channel bitstreaming work.
The changes for vdpau:

- drop depreciated ffmpeg struct vdpau_render_state and track video surfaces internally, implement buffer reget for ffmpeg
- only use a single vdpau device for all vdpau decoders. create/destroy of vdpau devices made life hard for the OSS devs.
- improved usage vdpau rendering pipeline
- fixed a bug when decoder was flushed
- drop old code path for pixmap magic. according to the OSS devs this was not compliant to the spec and did not work on AMD.