dropping the cached scaled bitmap when the bitmap
is accesed via BitmapAccessMode::Write for writing

Change-Id: Ib6539522944838238bd699ec3531039d21dc0f8b
Reviewed-on: https://gerrit.libreoffice.org/67459
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>