Fix for shaders.py based on code review comments from flokkievids.
Only the suggested changes that cause backward incompatibility were considered for now.

* Removed pyFXSThicknessShader that is identical with pyConstantThicknessShader.

* Swapped the order of two arguments of the pyDecreasingThicknessShader constructor
in line with other shaders taking the same arguments.

* Made module functions smoothC and get_fedge into methods of relevant shader classes.

* Removed pyExtremitiesOrientationShader that relied on undefined Stroke methods.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56482 954f8c5b-7b00-dc11-b283-0030488c597c