Blender Git Loki

Fix for Sinus Displacement and 2D Offset stroke geometry modifiers.

These modifiers were not working properly when they were applied to strokes
whose backbone was already modified by other geometry shaders. This problem
was due to the use of Normal2DF0D that compute 2D vertex normals based on
the underlying FEdges up on which initial stroke geometry is defined. Now vertex
normals are computed on the basis of modified stroke vertices.

A helper function 'stroke_normal' for computing normals of stroke vertices was
added to the 'freestyle.utils' API module.