Blender Git Loki

Fix T59505: Knife tool cut preview line becomes black

The root of the problem is that KnifeTool_OpData->colors was not init in
some cases. But the reason is unknown as it seems to be random and the
init function was always called.

So instead on init the color only once, we query the colors each time
we draw the knife points.
The overhead of this approach is negligeable.