Before, neither the texture index nor its semantic were kept in the
returned output. Now GetMaterialProperties returns a (name, color,
material, texture) tuple. Name is the name of the material. Color is a
dictionary of color properties. Material is a dictionary of material
properties. Textures is a triply nested dictionary addressed by the
following: textures[index][semantic][key]. See assimp documentation for
the meaning of index, semantic, and keys in general.
the strings used in materials and elsewhere in assimp. Because of this,
in PyAssimp3 strings in materials don't work properly on 64 bits
platforms. This commit adds a StringUInt32 ctypes structure and use it
in GetMaterialProperties as a workaround.