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/pointer.go

@@ -25,7 +25,9 @@ type OpHandler struct {
 	Grab bool
 }
 
-type Area func(pos f32.Point) HitResult
+type Area interface {
+	Hit(pos f32.Point) HitResult
+}
 
 type Key interface{}