Blender Git Loki

GPencil: Curvature support for length modifier

Extend the stroke following an approximated circluar/helical curve.

This can be used as an effect for lineart or on its own as helix generator.

Reviewed By: Sebastian Parborg (zeddb), Hans Goudey (HooglyBoogly), YimingWu (NicksBest), Antonio Vazquez (antoniov), Henrik Dick (weasel)

Differential Revision: https://developer.blender.org/D11668

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# ???? master
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# ??? source/blender/blenkernel/BKE_gpencil_geom.h
# ??? source/blender/blenkernel/intern/gpencil_geom.cc
# ??? source/blender/gpencil_modifiers/intern/MOD_gpencillength.c
# ??? source/blender/makesdna/DNA_gpencil_modifier_defaults.h
# ??? source/blender/makesdna/DNA_gpencil_modifier_types.h
# ??? source/blender/makesrna/intern/rna_gpencil_modifier.c
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