ImageOverlayPlugin: Adjusting the frame of the overlays results in an ever-increasing count

[gkjohnson]

When frame is adjusted the overlay is removed and added with the "forceDispose" flag set to "false". This lets the overlay texture draws get re-triggered without deleting all the underlying tiles but the "count" used for locking these tiles seems to continuously increase.

Other things to look at:

• Errors are blocked from being displayed (eg if no renderer is available then the error is caught and no logged on draw). Should check for the error type before blocking. Make a detectable error? Currently the log is "PriorityQueue: Item removed"
• _initTileOverlayInfo should derive initial level from the calculate level function