Calculate the frame deadline in ClutterFrameClock's
calculate_next_update_time_us() rather than in MetaWaylandCompositor's
on_after_update().

The specifics of the deadline calculation for a given frame should be
implementation detail of the frame clock and and remain internal to
allow extensibility.

This extensibility is specifically useful for scenarios where a
different deadline calculation is needed due to alternative frame
scheduling logic, such as for VRR.

No change in behavior.

Part-of: <GNOME/mutter!3521>