GstVdpOutputSrcPad is a subclass of GstPad that is used for pushing out
GstVdpOutputBuffers. It takes care of caps negotiation and conversion to
video/x-raw-rgb.
vdpausink is as the name implies a sink which takes vide/x-vdpau-output buffers
and outputs it to the screen.
vdpauvideopostprocess is an element which takes video/x-vdpau-video buffers and
outputs video/x-vdpau-output buffers. The element can also do things like
deinterlacing and other postprocessing but this is not implemented yet.
These elements are not fit for autoplugging, so demoting to RANK_NONE.
Also blacklisting for generic states test, since fixing these elements
up to do some minimal error handling seems to be a bit more work.
Partially fixes#591538.
