Switch underline font color if it is from the underline color
of the portion. If the underline color is auto, take the font
color into consideration.

Change-Id: I81a0e100cd024add603b574f07e10b5e3a785d0b
Reviewed-on: https://gerrit.libreoffice.org/41090Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mark Hung <marklh9@gmail.com>