GDKGLContext: Re-create an OpenGL/ES 3.0 context if needed

Hi,

This is a follow-up MR for MR !5829 (merged) so that we can attempt to create an OpenGL/ES 3.0 context to replace the OpenGL/ES 2.0 context that we have created previously by default, if mandatory extension(s) are not supported in the OpenGL/ES implementation, such as libANGLE, so that GTK can still run under OpenGL/ES mode.

libANGLE can be quite hard to track down since it is done in a rolling-release manner (which means large GIT checkouts), and currently the easiest way to get a working copy is via the QT 5.15.x sources, which is sadly a pretty old release series that do not yet support the needed extensions but does support OpenGL/ES 3.0.

This currently checks for the OES_vertex_half_float extension in the OpenGL/ES 2.0 implementation.

With blessings, thank you!