all: make pointer.Area an interface

With an interface instead of anonymous functions, amending an
area's parameters can be done even after adding it to an OpHandler.

This will be useful when we switch to serialized op lists.

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

ui/pointer/queue.go

@@ -101,7 +101,7 @@ func (q *Queue) opHit(handlers *[]Key, op ui.Op, pos f32.Point) (HitResult, bool
 			return HitNone, false
 		}
 		tpos := h.transform.InvTransform(pos)
-		res := h.area(tpos)
+		res := h.area.Hit(tpos)
 		if res != HitNone {
 			*handlers = append(*handlers, op.Key)
 		}
@@ -229,7 +229,7 @@ func (q *Queue) Push(e Event) {
 			e.Priority = Foremost
 		}
 		e.Position = h.transform.InvTransform(e.Position)
-		e.Hit = h.area(e.Position) != HitNone
+		e.Hit = h.area.Hit(e.Position) != HitNone
 		h.events = append(h.events, e)
 		if e.Type == Release {
 			// Release grab when the number of grabs reaches zero.