fix: touch input (#412)

`mouse-pos.hover` may or may not be false during touch input
(contrary to mouse input, where false means the cursor left the window)

Detecting a change to false as a leave event, while allowing any
other mouse-pos update to cause an enter event solves this.

scripts/uosc_shared/elements/Elements.lua

@@ -152,7 +152,7 @@ mp.set_key_bindings({
 		'mbtn_left',
 		Elements:create_proximity_dispatcher('mbtn_left_up'),
 		function(...)
-			update_mouse_pos(nil, mp.get_property_native('mouse-pos'), true)
+			update_mouse_pos(nil, mp.get_property_native('mouse-pos'))
 			Elements:proximity_trigger('mbtn_left_down', ...)
 		end,
 	},