Fix display creation code to check that any display obtained from a
neighbour actually has the type we expect. Note: if display type is
set to "any", we can then accept any VA display type.
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.
