Wrong handling of dragging when moving outside of game canvas area

[SteffenBauer]

Reported by 'EastEriq' from MobileRead Forum: "a node can be dragged below the drawing area and becomes invisible. The empty dot at the former position is no more a draggable handle; the node reappears once the screen is tapped somewhere else"

Bug possibly caused by missed 'mouse release' event when moving drag outside of game canvas.

Possible solution: Trigger release event when drag leaves game canvas.

[SteffenBauer]

Now triggering release event when initial tap was on canvas area.