Make gst_vaapi_display_get_display_type() return the actual VA display
type. Conversely, add a gst_vaapi_display_get_class_type() function to
return the type of the GstVaapiDisplay instance. The former is used to
identify the display server onto which the application is running, and
the latter to identify the original object class.
Add support for DRM Render-Nodes. This is a new feature that appeared
in kernel 3.12 for experimentation purposes, but was later declared
stable enough in kernel 3.15 for getting enabled by default.
This allows headless usages without authentication at all, i.e. usages
through plain ssh connections is possible.
Make sure to initialize one GstVaapiDisplay at a time, even in threaded
environments. This makes sure the display cache is also consistent
during the whole display creation process. In the former implementation,
there were risks that display cache got updated in another thread.
Make it possible to add extra an extra filter to most of display cache
lookup functions so that the GstVaapiDisplay instance can really match
a compatible and existing display by type, instead of relying on extra
string tags (e.g. "X11:" prefix, etc.).
Drop obsolete GST_VAAPI_IS_xxx() helper macros since we are no longer
deriving from GObject and so those were only checking for whether the
argument was NULL or not. This is now irrelevant, and even confusing
to some extent, because we no longer have type checking.
Note: this incurs more type checking (review) but the libgstvaapi is
rather small, so this is manageable.
