sanm: fix undefined behaviour on big-endian.

A variable with post-increment may only appear
once in a statement.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
vdpau_internal.h: Add missing include for FF_API_BUFS_VDPAU.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
vdpau: Add an allocation function for AVVDPAUContext.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>