To properly handle subsetted BitmapDevices in the iOS vcl backend I
seem to need to know what the size of the full BitmapDevice is.

I wasted at least one day on desperate hacking and debugging, trying
to wrap my head around a misunderstanding of what a subsetted
BitmapDevice is. I thought it involved coordinate offsetting...

Change-Id: I83bf1a7d75ce192aaf21f1e408008e362fd6c6e6