Added debug prints.
Removed ad-hoc use of R_PERSISTENT_DATA for Freestyle/Cycles.
Fix for nested Cycles sessions.
Fix for hard-coded #include file paths.
Removed debug prints.
Added new operator "Create Freestyle Stroke Material" for testing.
Always use the Blender Internal renderer for stroke rendering.
Freestyle: Show the Freestyle Line panel of the Material properties context also in Cycles
Implemented a basic framework for node-based shaders.

Shader nodes will be used to define materials for stroke rendering, so as to
allow users to interactively configure textures.
Enabled USE_CYCLES_FOR_STROKE_RENDERING compile-time flag by default.
Minor fix for texture category name.
Added new Output Line Style shader node for defining a node-based line material.
Fix for a wrong ID datablock referenced due to a typo in BKE_copy_linestyle().
Changed ntreeCopyTree_ex() to accept a pointer to struct Main.
Freestyle: Added preliminary support for textured strokes in Cycles.

Now the shader node tree of a line style ID datablock is used to define textures
as well as their mapping and influence.

TODO: Textures alpha channel mapping and influence.
TODO: Blend mode in the Output Line Style shader node.
Follow-up to rB647969f9b617: Renamed CTX_data_linestyle_from_scene() to BKE_get_linestyle_from_scene().
Freestyle: Add BKE_linestyle_default_shader() for creating the default line style shader node tree.

Changes to ED_node_shader_default() were reverted since the code there was actually
not suitable for setting up the default line style node tree properly.
Freestyle: Changed the default Color value of the Output Line Style node.
Freestyle: Fix for stroke_shader node default values copied from the Output Line Style node.
Freestyle: Fix for a crash in BKE_linestyle_default_shader() due to a typo.