WIP tiled backing store implementation

* Unused, enabling this needs some tweaking of the CMakeLists.
* May or may not be better than our current big offscreen bitmap.
Abort requests early when there is no handler.

* This may happen when a request is launched, then cancelled.
* Avoids use of deleted object.
Avoid String > BString > String conversion.

* Not that it matters much, but it's cleaner this way.