io/pointer: add Drag event type

This eliminates needless redraws for handlers that care about drag events and not move events, like gesture.Scroll.

Signed-off-by: Gordon Klaus <gordon.klaus@gmail.com>

io/pointer/pointer.go

@@ -92,6 +92,8 @@ const (
 	Release
 	// Move of a pointer.
 	Move
+	// Drag of a pointer.
+	Drag
 	// Pointer enters an area watching for pointer input
 	Enter
 	// Pointer leaves an area watching for pointer input
@@ -180,6 +182,8 @@ func (t Type) String() string {
 		return "Cancel"
 	case Move:
 		return "Move"
+	case Drag:
+		return "Drag"
 	case Enter:
 		return "Enter"
 	case Leave: