Slim GdkGLContext APIs

All rendering requires gl contextes sharing the paint context, so
make gdk_window_create_gl_context always share that and remove the
_shared version (can be added back later if something really needs it).

Also make gdk_window_get_paint_gl_context internal as it is no longer
needed in the public API.

Also remove gdk_gl_context_update() which is not needed any more.
Update docs for the GL stuff