Freestyle: Hooks for custom pre/post-processing line stylization procedures.
This patch adds three lists of callback functions for Freestyle Parameter Editor to
allow users to modify the Freestyle line rendering procedure.
- lineset_pre: functions in this list are called back just before a line set is going to be processed.
- base_style_post: functions are called back after base line stylization is done.
- lineset_post: functions are called back just after line set processing is done.
All callbacks receive as arguments the current scene, render layer and line set.
Callbacks for lineset_pre and lineset_post can perform whatever operations
allowed for stand-alone style modules for the Python Scripting mode.
Callbacks for base_style_post have to return a list of stroke shaders (i.e.,
instances of StrokeShader subclasses).