plugins: create a GL context on certain conditions

plugins: create a GL context on certain conditions

If a GstVaapiDisplay is not found in the GStreamer context sharing,
then VAAPI elements look for a local GstGLContext in gst context
sharing mechanism ('gst.gl.local.context').

If this GstGLContext not found either then, only the VAAPI decoders
and the VAAPI post-processor, will try to instantiate a new
GstGLContext.

If a valid GstGLContext is received, then a new GstVaapiDisplay will
be instantiated with the platform, API and windowing specified by the
instantiated GstGLContext.

Original-Patch-By: Matt Fischer <matt.fischer@garmin.com>
https://bugzilla.gnome.org/show_bug.cgi?id=777409