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().