Changed how buttons are stored in press and release events

examples/draw/main.go

@@ -38,16 +38,16 @@ func main () {
 			os.Exit(0)
 
 		case stone.EventPress:
-			event := event.(stone.EventPress)
-			if stone.Button(event) == stone.MouseButtonLeft {
+			button := event.(stone.EventPress).Button
+			if button == stone.MouseButtonLeft {
 				mousePressed = true
 				application.SetRune(0, 0, '+')
 				application.Draw()
 			}
 
 		case stone.EventRelease:
-			event := event.(stone.EventRelease)
-			if stone.Button(event) == stone.MouseButtonLeft {
+			button := event.(stone.EventRelease).Button
+			if button == stone.MouseButtonLeft {
 				mousePressed = false
 				application.SetRune(0, 0, 0)
 				application.Draw()