[0.92.x] Ensure LPE stroke retains colour when 'Triangle In' shape used

This cherry picks commit d0930c9b from master to fix the following bug:

LP#1707899: Bezier Curve does not retain colour used when Triangle In shape is used

0.92.x backport was requested at the time for by @jabiertxof, would like to make sure there were no problems with this commit.