There's pretty much the same code in CanvasHelper::fillTexturedPolyPolygon()
that has received various fixes over the time, but this code not, so
fix it as well.

Change-Id: I7293d4d67dff2d5276928bb3ab25adfb883ce3ca
Reviewed-on: https://gerrit.libreoffice.org/72700
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>