Blender Git Loki

Made stroke rendering stability fixes, with the aim of addressing
occasional unexpected long lines.

1. The Parameter Editor mode was extended to prevent strokes from
doing quick U-turns that "enable" a known bug in strip creation
that generates unexpected long lines in question.

2. A verbose warning message was added to make the existence of
the strip creation bug visible to users. When the bug affects the
stroke rendering, the following warning shows up in the console:

> Warning: problem in strip creation (the strip is most likely doing a U-turn).

3. The extrapolation option of CurveMapping (used in alpha and
thickness modifiers in the Parameter Editor mode) was identified
as another source of unexpected long lines. Now the extrapolation
option is unconditionally disabled (even when users enable it
through the GUI).