Impress core only support a single step, improve the conversion from
multi-step to single step to take transparency into account explicitly.

Once we select the widest segment, look backwards and forward if there
are other next segments which have the same RGB color, just different
transparency and include them. This helps in general, because in case a
0% or 100% transparency is mishandled, that's very visible.

Change-Id: I11d593c01a6a4b16149ce74c1408c2a39895e941
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/100231
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins