Render setup: Add support for single sided polygons.

Note that shader node trees are cleared every time shading is switched or reset.
For now, it works only with "GLSL" or "Shadeless".
PMX importer: Set toon textures to texture slots, needed for toon shading support.

- In texture slots, unused bmp image texture with 32x32 pixels is considered as a toon texture
- Add toon_textures module that includes shared texture generator
- SharedTexture class in pmx module is not used (Maybe it can be deleted)
- Also fixes file path issues on Linux and OSX (case-sensitivity and path separators)