[backend] Abstract the GL context creation

This is the another step into abstracting the backend operations
that are currently spread all across the board back into the
backend implementations where they belong.

The GL context creation, for instance, is demanded to the stage
realization which makes it a critical path for every operation
that is GL-context bound. This usually does not make any difference
since we realize the default stage, but at some point we might
start looking into avoiding the default stage realization in order
to make the Clutter startup faster.

It also makes the code maintainable because every part is self
contained and can be reworked with the minimum amount of pain.

clutter/clutter-private.h

@@ -184,6 +184,9 @@ ClutterActor *_clutter_backend_create_stage   (ClutterBackend  *backend,
                                                GError         **error);
 void          _clutter_backend_ensure_context (ClutterBackend  *backend,
                                                ClutterStage    *stage);
+gboolean      _clutter_backend_create_context (ClutterBackend  *backend,
+                                               gboolean         is_offscreen,
+                                               GError         **error);
 
 void          _clutter_backend_add_options    (ClutterBackend  *backend,
                                                GOptionGroup    *group);