Blender Git Loki

GP Stroke Sculpting: Initial implementation of a "Smooth" Brush

This commit introduces a basic "smooth" brush tool for sculpting Grease Pencil
strokes (i.e. for getting rid of wobbles or kinks in the stroke). The current
implementation is still quite rough (with quite a few points noted about how it
could be better).

Usage Notes:
* It is highly recommended to have the "Use Falloff" option enabled. Doing so
allows you to have quite a bit more subtlety to how well you can control the
effects of this brush (i.e. this option can be used to prevent the brush from
"shrinking" the strokes too much, if you use the edge of the brush)

* A radius of 25 px, and strength = 0.5 are also good starting points


Implementation Notes:
* This currently only affects the stroke coordintes. Whether pressure values should
be handled by this brush (with/without an option to turn this on/off), or whether
it should be handled as a separate brush remains to be seen.

* This isn't exactly the most efficient implementation. It also suffers from the
fact that because its effects are applied from the first point in the stroke
to the last (and in a single pass), you can get unwanted accumulated smoothing
effects.

* Currently all points in the neighbourhood are given equal priority. We could
investigate different weighting schemes here.

* This is currently hardcoded to consider 2 points before and 2 points after the
current one. This could be improved by offering control over how many on either
side to consider...