...and rename fBitmap to fOffscreenBitmap to make it more clear what it is.
We don't need to save a pointer to both the offscreen bitmap and
the offscreen view, just the bitmap. We can access the view by calling
fOffscreenBitmap->ChildAt((int32)0). This gives us back a _reserved private
variable slot.
In the (unlikely) case that _InitData() is called with offscreen = false but
the fOffscreenBitmap is not NULL, delete fOffscreenBitmap before
setting it to NULL so that memory is not leaked.