devdraw: respond to windowDidBecomeKey on darwin (#239)

Fixes bug where devdraw does not "notice" mouse position after task
switch. Fixes https://github.com/9fans/plan9port/issues/232.

src/cmd/devdraw/cocoa-screen-metal.m

@@ -373,6 +373,11 @@ struct Cursors {
 	return YES;
 }
 
+- (void)windowDidBecomeKey:(id)arg
+{
+        [myContent sendmouse:0];
+}
+
 @end
 
 @implementation DevDrawView