In addition to D319, this patch updates the parameter editor, the UI of Freestyle.
Using new API functionality and experience gained in making D319, this patch
provides a quite noticable speedup for commonly-used Freestyle linestyle modifiers.
As this patch touches a lot of code (and mainly the foundations) it is likely that
mistakes are made. The patch has been tested with a regression suite for Freestyle
(https://github.com/folkertdev/freestyle-regression-tests/tree/master), but testing
with scenes used in production is very much appreciated.
Differential revision: https://developer.blender.org/D623
Author: flokkievids (Folkert de Vries)
Reviewed by: kjym3 (Tamito Kajiyama)
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