io/router: simplify pointer event routing

- Drop pointer.Event.Hit in favour of Enter/Leave events.
- Track enter/leaves for each pointer.ID (updates #122). Add test.
- Resolve grabs once.
- Get rid of scratch slice.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gesture/gesture.go

@@ -145,7 +145,7 @@ func (c *Click) Events(q event.Queue) []ClickEvent {
 		case pointer.Cancel:
 			c.state = StateNormal
 		case pointer.Press:
-			if c.state == StatePressed || !e.Hit {
+			if c.state == StatePressed {
 				break
 			}
 			if e.Source == pointer.Mouse && e.Buttons != pointer.ButtonLeft {