Having it subclass Bitmap encourages confusion in passing it around, and
I need the extra type-safety for my work on merged-alpha
Change-Id: I35819f9b8ee609cbdaf865563c78531e397b529b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/160235
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>