tdf#128345 PPTX: add transparence gradient for fill in chart

[Regina Henschel]

Hi all,

the patch
   https://gerrit.libreoffice.org/c/core/+/99464
   tdf#128345 PPTX: add transparence gradient for fill in chart
brings transparency gradients to charts. It is a follow up to
   https://gerrit.libreoffice.org/c/core/+/98792
   tdf#128345 pptx export: add transparence gradient in solid fill

If you want to review it, please add yourself in 99464. Otherwise I will 
submit it tomorrow.

Kind regards
Regain