gdk: Add GdkDisplay::init_gl vfunc

gdk: Add GdkDisplay::init_gl vfunc

The vfunc is called to initialize GL and it returns a "base" context
that GDK then uses as the context all others are shared with. So the GL
context share tree now looks like:

+ context from init_gl
  - context1
  - context2
  ...

So this is a flat tree now, the complexity is gone.

The only caveat is that backends now need to create a GL context when
initializing GL so some refactoring was needed.

Two new functions have been added:

* gdk_display_prepare_gl()
  This is public API and can be used to ensure that GL has been
  initialized or if not, retrieve an error to display (or debug-print).
* gdk_display_get_gl_context()
  This is a private function to retrieve the base context from
  init_gl(). It replaces gdk_surface_get_shared_data_context().