devdraw: add forcedpi toggled by Fn+F3 on Mac

R=rsc
http://codereview.appspot.com/6846104

src/cmd/devdraw/cocoa-srv.c

@@ -367,6 +367,8 @@ abortcompose(void)
 		keystroke(Kalt);
 }
 
+void resizeimg(void);
+
 void
 keystroke(int c)
 {
@@ -379,6 +381,16 @@ keystroke(int c)
 		nk = 0;
 		return;
 	}
+	if(c == KF+3) {
+		if(forcedpi)
+			forcedpi = 0;
+		else if(displaydpi >= 200)
+			forcedpi = 100;
+		else
+			forcedpi = 225;
+		resizeimg();
+		return;
+	}
 	if(!alting){
 		kputc(c);
 		return;