fdo#65295 : swap gradient text-box colors only in case of axis

This commit changes the behavior of the export process of text-box
gradient color. Currently it exports the colors inverted,
but it should do so only when it is an 'axis' gradient.

Change-Id: I8841982c1dbc57616d6e1640d73bc8aa823ae056
Reviewed-on: https://gerrit.libreoffice.org/6646
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>