gdk: Add missing GDK_DEBUG token
gdk: Rework the GL context update

Ensure that gdk_gl_context_update() schedules an update of the viewport,
and then perform the actual update when the backend says it should. This
makes the code a bit more readable, and we can keep asking the user to
call gdk_gl_context_update() after gdk_window_move_resize() without
having to care about asynchronous updates.
tests/glarea: Remove explcit glFlush()

It's really not needed.
glarea: Unconditionally call gdk_gl_context_update()

There's no need to check for different sizes.
gdk/x11: Add API to query the GLX version
docs: Add some more documentation to GdkGLContext
gdk: Add licensing notes for GdkGL*
gtk: Document GtkGLArea
gdk: Remove unused function

The clear_window() function is replaced by the set_window() function
called with a NULL window.
docs: Add Gdk OpenGL API to the reference
docs: Add GtkGLArea to the API reference