Set the event for group marker selection to the last real drawcall
Add quad overdraw mode. Thanks Stephen Hill (@self_shadow)!

* http://blog.selfshadow.com/2012/11/12/counting-quads/
* Quad Overdraw based on ScenePS4 from the revised implementation. The
  colours are new, up to 20 levels. Picking pixels shows the overdraw level
* Available per-pass and per-drawcall, a pass defined the same way as in
  the mesh view, drawcalls since last clear or RT change.
* List of events now passed through to RenderOverlay the same as RenderMesh