Removed ValuePlug::Computation::resultWritten.

It dates back to a time when the results of computations were allowed to be NULL - since they are now guaranteed to be non-NULL we can just test m_resultValue to see if it has been written.
Fixed threading issue in ValuePlug caching code.

We were calling cached() and then calling get(), but in the moments in between, another thread could have removed the item we wanted from the cache. The new test cases would either throw exceptions from the nullGetter() or they would hang - now they don't.
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 #689 from johnhaddon/valuePlug

Cache threading fix
Merge pull request #692 from johnhaddon/overlayDragFix

Overlay mouse position fixes