Distinguish "no delay" frames from spontaneous drawing

When a client is drawing as hard as possible (without sleeping
between frames) we need to draw as soon possible, since sleeping
will decrease the effective frame rate shown to the user, and
can also result in the system never kicking out of power-saving
mode because it doesn't look fully utilized.

Use the amount the client increments the counter value by when
ending the frame to distinguish these cases:

 - Increment by 1: a no-delay frame
 - Increment by more than 1: a non-urgent frame, handle normally

https://bugzilla.gnome.org/show_bug.cgi?id=685463