vdpau: common support for managing the VdpDecoder in avcodec

Using the not so new init and uninit callbacks, avcodec can now take
care of creating and destroying the VDPAU decoder instance.

The application is still responsible for creating the VDPAU device
and allocating video surfaces - this is necessary to keep video
surfaces on the GPU all the way to the output. But the application
will no longer needs to care about any codec-specific aspects.

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