Improve (especially remote) window dragging and resizing performance

Before this change, every window operation with the potential to affect the _NET_WM_STATE property would end up calling XChangeProperty(), even if the property didn't actually change. This would cause an avalanche of PropertyNotify events, to which GTK-based apps, at least, dutifully respond with a GetProperty request. The connection is then overwhelmed with these requests.

The fix made here is to remember the last value of _NET_WM_STATE that was set, returning early if the new value is equal to the last value.

If you want to see the impact of this change for yourself, try ssh -Y-ing into a remote host, running the gtk3-demo app (available in the gtk-3-examples apt package), and dragging the window around for a bit. Without the change, the window will hang for seconds at a time and lag behind the cursor significantly. With the change, at least for me over wi-fi, the window keeps up with the cursor acceptably, without significant lag or hanging.

changed title from Improve remote window dragging and resizing performance to Improve (especially remote) window dragging and resizing performance

requested review from @jeremyhu

approved this merge request

merged

mentioned in commit babff9d7