Fix "is mount_event" check in gui_in_3d to work with both 4 beta and 4 rc

plugins/addons/main_screen/main_screen_plugin.gd

@@ -29,7 +29,7 @@ func _make_visible(visible):
 
 # If your plugin doesn't handle any node types, you can remove this method.
 func _handles(object):
-	return object is preload("res://addons/main_screen/handled_by_main_screen.gd")
+	return is_instance_of(object, preload("res://addons/main_screen/handled_by_main_screen.gd"))
 
 
 func _get_plugin_name():

viewport/gui_in_3d/gui_3d.gd

@@ -36,7 +36,7 @@ func _unhandled_input(event):
 	# Check if the event is a non-mouse/non-touch event
 	var is_mouse_event = false
 	for mouse_event in [InputEventMouseButton, InputEventMouseMotion, InputEventScreenDrag, InputEventScreenTouch]:
-		if event is mouse_event:
+		if is_instance_of(event, mouse_event):
 			is_mouse_event = true
 			break