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D545: Freestyle Python API: new methods for Stroke and StrokeVertexIterator.

This revision extends the Freestyle Python API to make for style module writing
easier.

- freestyle.types.Stroke: A proper support for reversed() is implemented. It
works the same with other Python sequence objects (returns an iterator starting
from the end). This is in effect equivalent to Stroke.stroke_vertices_end().

- freestyle.types.StrokeVertexIterator: An incremented, decremented and reversed
method are added. The first two methods return a new StrokeVertexIterator
object that has been incremented and decremented, respectively. The reversed
method returns a new StrokeVertexIterator object that will traverse stroke
vertices in the opposite direction.

- freestyle.types.Interface0DIterator: Its constructor now accepts a Stroke
object to create an Interface0DIterator that traverses stroke vertices. This is
in effect equivalent to Stroke.vertices_begin(). The new API makes stroke
shaders involving function calls much simpler as illustrated below:

# in the old API
it = stroke.stroke_vertices_begin()
for vert in it:
result = somefunc(Interface0DIterator(it))

# in the new API
it = Interface0DIterator(stroke)
for vert in it:
result = somefunc(it)


Differential Revision: https://developer.blender.org/D545

Reviewers: kjym3