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
