cmd/devdraw: clear keyboard state on lost focus.

See https://bitbucket.org/rsc/plan9port/issue/128/alt-button-sticks-in-acme-sometimes-after

R=rsc
https://codereview.appspot.com/11453043

src/cmd/devdraw/x11-srv.c

@@ -619,6 +619,12 @@ runxevent(XEvent *xev)
 		break;
 	
 	case FocusOut:
+		/*
+		 * Some key combinations (e.g. Alt-Tab) can cause us
+		 * to see the key down event without the key up event,
+		 * so clear out the keyboard state when we lose the focus.
+		 */
+		kstate = 0;
 		abortcompose();
 		break;