The fill tool's stroke extension feature has a "radius" mode that was
implemented by just drawing filled circles at the end of strokes.
This should work by creating additional lines instead, the circles are
just indicators of the search range.
The radius mode now does a KDTree search for points inside the circles.
It generates extra extension lines and handles these in the usual way,
just like lines generated in "extension" mode.
GPv2 had an undocumented curvature feature that would also add lines to
interior points if their curvature was large enough. This is quite
obscure, not mentioned in the manual, and adds complexity. For now the
feature is not implemented, but some placeholder TODO comments have been
added.
Pull Request: https://projects.blender.org/blender/blender/pulls/125420
418c63f8db