tdf#151547 pptx import: workaround for COL_AUTO collision
In the current implementation of ::Color, it is not possible
to have fully transparent white text (since it collides with
COL_AUTO and gets interpreted as Automatic Color).
Implement a workaround for import of fully transparent white
text color so that it isn't interpreted as the magic value
COL_AUTO (i.e. instead of fully transparent #FFFFFF import
as fully transparent #FFFFFE).
Change-Id: Ide750093ef8a89f1424ddd8f4e9ee1e18209f2ad
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/141439
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>