Adds texture type enum to pyassimp

Keeping the same style of `postprocess.py`, this is a port of the aiTextureType enum in [`material.h`](https://github.com/assimp/assimp/blob/master/include/assimp/material.h).

port/PyAssimp/pyassimp/material.py

@@ -0,0 +1,89 @@
+## <hr>Dummy value.
+#
+# No texture, but the value to be used as 'texture semantic'
+# (#aiMaterialProperty::mSemantic) for all material properties
+# *not* related to textures.
+#
+aiTextureType_NONE = 0x0
+
+## <hr>The texture is combined with the result of the diffuse
+# lighting equation.
+#
+aiTextureType_DIFFUSE = 0x1
+
+## <hr>The texture is combined with the result of the specular
+# lighting equation.
+#
+aiTextureType_SPECULAR = 0x2
+
+## <hr>The texture is combined with the result of the ambient
+# lighting equation.
+#
+aiTextureType_AMBIENT = 0x3
+
+## <hr>The texture is added to the result of the lighting
+# calculation. It isn't influenced by incoming light.
+#
+aiTextureType_EMISSIVE = 0x4
+
+## <hr>The texture is a height map.
+#
+# By convention, higher gray-scale values stand for
+# higher elevations from the base height.
+#
+aiTextureType_HEIGHT = 0x5
+
+## <hr>The texture is a (tangent space) normal-map.
+#
+# Again, there are several conventions for tangent-space
+# normal maps. Assimp does (intentionally) not
+# distinguish here.
+#
+aiTextureType_NORMALS = 0x6
+
+## <hr>The texture defines the glossiness of the material.
+#
+# The glossiness is in fact the exponent of the specular
+# (phong) lighting equation. Usually there is a conversion
+# function defined to map the linear color values in the
+# texture to a suitable exponent. Have fun.
+#
+aiTextureType_SHININESS = 0x7
+
+## <hr>The texture defines per-pixel opacity.
+#
+# Usually 'white' means opaque and 'black' means
+# 'transparency'. Or quite the opposite. Have fun.
+#
+aiTextureType_OPACITY = 0x8
+
+## <hr>Displacement texture
+#
+# The exact purpose and format is application-dependent.
+# Higher color values stand for higher vertex displacements.
+#
+aiTextureType_DISPLACEMENT = 0x9
+
+## <hr>Lightmap texture (aka Ambient Occlusion)
+#
+# Both 'Lightmaps' and dedicated 'ambient occlusion maps' are
+# covered by this material property. The texture contains a
+# scaling value for the final color value of a pixel. Its
+# intensity is not affected by incoming light.
+#
+aiTextureType_LIGHTMAP = 0xA
+
+## <hr>Reflection texture
+#
+#Contains the color of a perfect mirror reflection.
+#Rarely used, almost never for real-time applications.
+#
+aiTextureType_REFLECTION = 0xB
+
+## <hr>Unknown texture
+#
+# A texture reference that does not match any of the definitions
+# above is considered to be 'unknown'. It is still imported
+# but is excluded from any further postprocessing.
+#
+aiTextureType_UNKNOWN = 0xC