Fixed bug in Widget.mousePosition() with GLWidget overlays.

Qt's mapFromGlobal() method doesn't do the right thing when the widget is in a GraphicsView, as it is in a GLWidget overlay. We've already dealt with that issue in Widget.bound(), so rejigging the relativeTo behaviour in terms of that fixes things.
Fixed drag/drop offset in GLWidget overlays.

The reported mouse position was being offset because we were relying on Qt's mapFromGlobal() method which doesn't work properly for widgets inside QGraphicsScenes. This affected the virtual sliders for image viewer exposure/gamma widgets.
Merge pull request #692 from johnhaddon/overlayDragFix

Overlay mouse position fixes