fdo#82859 RTF import: fillColor is unsigned

Otherwise ARGB 0xFFFFFFFF would turn into black.

(cherry picked from commit 04ea7b24ec1b5a027efa0b850f2bc3ac7116c52e)

Change-Id: I690f34d888ca9a013e8ce1af2c7ef8ed88ee28b0
Reviewed-on: https://gerrit.libreoffice.org/12153
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>