Fix screen cast cursor metadata with unthrottled input

From the main commit:

With the unthrottled input emission, we ended up often getting the cursor updates long before any damage had been posted, meaning that if you moved around the mouse pointer where the mouse had a high enough refresh rate, we'd effectively stall the screen cast stream by only sending cursor updates and nothing else.

Fix this by scheduling an update when we get a cursor update, then sending a cursor-only frame after any damage and relayout has been processed, but only if there is no queued damage that will cause an actual repaint.