devdraw: clear altdown on focus out (thanks Ethan Burns)

TBR=rsc
https://codereview.appspot.com/53820044

src/cmd/devdraw/x11-srv.c

@@ -625,6 +625,7 @@ runxevent(XEvent *xev)
 		 * so clear out the keyboard state when we lose the focus.
 		 */
 		kstate = 0;
+		altdown = 0;
 		abortcompose();
 		break;