Don't disarm deadline timer when starting to composite
If we finish compositing in time, the composited result will be submitted prior to the deadline timer is triggered, and we'll be fine, and if not, at least the cursor updates will be smooth, which makes it appear smoother than not.
There is a risk that this can negatively impact composited updates when moving the cursor, so make it possible to toggle a paint-debug flag for now until this has been more tested.
This also mean we need to disarm the deadline timer after handling update, as there might be a scheduled cursor update pending, but we already handled it, so disarm the timer.