First plan was to move it to bitmapaccess.hxx, but that would add
dependency for the backends, which only need BitmapAccessMode.

Change-Id: Id8ed980207da5b0c7d4de48d82446b33f6b76b13
Reviewed-on: https://gerrit.libreoffice.org/70827
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>