clutter/stage: Cache get_update_time results
Using a 1000Hz mouse, descending into _clutter_stage_get_window
and _clutter_stage_window_get_update_time
was starting to become
expensive (10% of gnome-shell's CPU time) due to various
g_type_check_..
calls. So cache the results where it makes sense
to do so.
Related to: #283 (closed)