bugfix: each data marker on a doughnut as a different color
bugfix: each data marker on a doughnut as a different color

Doughnut charts display a single "Serie 1" label in their legend on
OpenOffice. This is due to `varyColor` (or "Vary Colors By Point") being
set to 0 on them. Since doughnut charts always have one color per
"point" (area), this should always be set to 1 for them.

gitbrent/PptxGenJS,gitbrent/PptxGenJS,gitbrent/PptxGenJS