vdpau: Add context and common helpers for hwaccel support

Signed-off-by: Diego Biurrun <diego@biurrun.de>
vdpau: Add MPEG-1/2 decoding via hwaccel infrastructure

Signed-off-by: Diego Biurrun <diego@biurrun.de>
vdpau: Add MPEG-4 decoding via hwaccel infrastructure

Signed-off-by: Diego Biurrun <diego@biurrun.de>
vdpau: Add H.264 decoding via hwaccel infrastructure

Signed-off-by: Diego Biurrun <diego@biurrun.de>
vdpau: Add VC-1 decoding via hwaccel infrastructure

Signed-off-by: Diego Biurrun <diego@biurrun.de>