vdpau: add av_vdpau_bind_context()

vdpau: add av_vdpau_bind_context()

This function provides an explicit VDPAU device and VDPAU driver to
libavcodec, so that the application is relieved from codec specifics
and VdpDevice life cycle management.

A stub flags parameter is added for future extension. For instance, it
could be used to ignore codec level capabilities (if someone feels
dangerous).

Signed-off-by: Anton Khirnov <anton@khirnov.net>