Freestyle: new stroke modifiers

Freestyle: new stroke modifiers

This patch introduces a couple new stroke modifiers. The ones currently implemented are based on prototypes by @kjym3 and myself.

The new modifiers:
  - Tangent
  - Thickness noise
  - Crease Angle
  - Simplification
  - Curvature 3D

The documentation for these new modifier types can be found [[ http://www.blender.org/manual/render/freestyle/parameter_editor/index.html | in the manual ]]:

{F134441}
(left: AnisotropicThicknessShader, right: NoiseThicknessShader)

{F140499}
(left: Curvature 3D, right: Simplification)

Author: Folkert de Vries (flokkievids)

Reviewers: kjym3

Subscribers: #user_interface, plasmasolutions, kjym3

Projects: #bf_blender

Differential Revision: https://developer.blender.org/D963