Drawing code `paint_draw_smooth_cursor` would be called correctly, it
was just the color not being initialized.
This is usually done with `BKE_paint_init`, but in the case of curves
sculpting brushes this would create an additional (unnamed) brush which
should be avoided since the workspace toolsystem creates the "right"
brush anyways.
So this patch just does the minimal work to get the Stabilize Stroke
indicator line to draw (which is initializing the color).
Brought over from https://archive.blender.org/developer/D16793
Philipp Oeser