Add GstVaapiTextureEGL abstraction that can create its own GL texture,
or import a foreign allocated one, while still allowing updates from a
VA surface.
Add initial support for EGL to libgstvaapi core library. The target
display server and the desired OpenGL API can be programmatically
selected at run-time.
A comprehensive set of EGL utilities are provided to support those
dynamic selection needs, but also most importantly to ensure that
the GL command stream is executed from within a single thread.
https://bugzilla.gnome.org/show_bug.cgi?id=743846
