Factor out propose_allocation() hooks, creation of video buffer pool
for the sink pad, conversion from raw YUV buffers to VA surface backed
buffers. Update vaapidecode, vaapiencode and vaapipostproc to cope
with the new GstVaapiPluginBase abilities.
Move common VA display creation code to GstVaapiPluginBase, with the
default display type remaining "any". Also add a "display-changed"
hook so that subclasses could perform additional tasks when/if the
VA display changed, due to a new display type request for instance.
All plug-ins are updated to cope with the new internal APIs.
Introduce a new GstVaapiPluginBase object that will contain all common
data structures and perform all common tasks. First step is to have a
single place to hold VA displays.
While we are at it, also make sure to store and subsequently release
the appropriate debug category for the subclasses.
