Fix mousemove handle on windows

Author: mhochsteger

webgpu/input_handler.py

@@ -54,7 +54,7 @@ def __on_mouseup(self, ev):
 
     def __on_mousemove(self, ev):
         self._is_moving = True
-        if ev["buttons"] != 0:
+        if "buttons" in ev and ev["buttons"] != 0:
             self.emit("drag", ev)
 
     def on(