Sometimes, there may be motion events left in the queue, because at the
time of handlign these motion events the GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS
phase is not requested.

Ensure that, when finding motion events on a window that does not disable
motion compression, the events are anyways flushed, so they can be processed
early by the window/widget event handlers.

Closes: #4946