OS X: update mouse position on mouse down
Created by: underdoeg
Without this fix, godot becomes unusable with an iiyama touchscreen on OS X.
On OS X godot only updates the mouse position on mouse move. But on a touchscreen there is no mouse move preceding a mouse down event. So the old (cached) mouse move position is used to trigger the touch down event. This results in irratical touch down and up behaviours. This PR makes sure the mouse position is updated from the OS before feeding the event into the Input system.