expose more flutter framework performance toggles

- PerformanceOverlayLayer.checkerboardOffscreenLayers
- PerformanceOverlayLayer.checkerboardRasterCachedImages

@kenzieschmoll want to add these?
I'd propose what we should do for experimental buttons is to add them to devtools, track usage statistics and if there is usage add them to IntelliJ as well.
In Devtools the window is wider and we can provide descriptions of each button so users can get a better idea what the buttons do.