Redraw window at regular intervals not a fixed time after last move

[yourealwaysbe]

I noticed that with fractional scaling on Wayland scrolling with the trackpad became very laggy. This pull request fixes that. Hopefully in a reasonable way.

Previously, each redraw would start a timer before the update took place. Each time a new event came, the timer would be reset. This means the update can be indefinitely delayed by a long sequence of events.

Instead, only set the timer if it doesn't exist, so an update is guaranteed to happen at regular intervals, rather than waiting for a pause in the event stream.