Blender Git Loki

New options in the Parameter Editor mode for controling the position of stroke thickness.

The new options enable a better control on the position of stroke thickness with
respect to stroke backbone geometry. Three predefined positions are:
* center: thickness is evenly split to the left and right side of the stroke geometry.
* inside: strokes are drawn within object boundary.
* outside: strokes are drawn outside the object boundary.
Another option called "relative" allows users to specify the relative position by a
number between 0 (inside) and 1 (outside).

The thickness position options are applied only to strokes of the edge types SILHOUETTE
and BORDER, since these are the only edge types defined in terms of object boundary.
Strokes of other edge types are always using the "center" option.