clutter/frame-clock: Avoid rapidly toggling dynamic max render time
This could happen when moving the cursor over GUIs that only redraw
in response to cursor movement. Mutter would experience alternating
cursor-only updates and page flips, and so the max_render_time_allowed_us
would jump between pessimised and optimised resulting in inconsistent
frame pacing.
Aside from fixing the smoothness problem this should also provide lower latency cursor movement.