Support multiple diffuse textures

[Tarcontar]

At the moment the plugin does not support multiple diffuse textures defined in materials. (e.g. Texture_0 and Texture_1 defined in FxShader/ShaderMaterial). This is due to the fact, that the currently supported shader node setup for import and export via python looks sth like that:

See https://docs.blender.org/manual/fr/dev/addons/import_export/io_node_shaders_info.html for more information.

[Tarcontar]

In w3x there might also be additional uv coordinates for the second texture.