Export Material Data as .json

[Mets3D]

This PR exports .json material data whenever you export a model.

While most of this data may be useless or yet-to-be-deciphered by the community, here are some of my observations:

• "TextureMaps" obviously contains the textures.
  • "SamplerName" gives a clue as to the use of the texture. Switch Toolbox's best guess is found in the "Type" property. (eg. a sampler of "_a0" corresponds to the type "Diffuse". But at the end of the day the latter is just a guess, and the former is the true data from the game itself.)
• "isTransparent" is always True for materials which should be transparent, but also in some cases where they shouldn't. It's never False for materials that should be transparent.
• texture_array_index properties seem to indicate which terrain texture to use. Sometimes a material uses more than one terrain texture, and it might use the vertex colors or those edges of the mesh which only have 1 face, to blend between them.
• uking_texture_array_texture properties seem to have something to do with these terrain textures as well, but the exact correlation is hard to gauge. A value of -1 probably means that the corresponding value in texture_array_index can be ignored (so a value of 0 there wouldn't be referring to the 0th terrain texture, but instead it should just be ignored.)
• RenderState -> _flags is probably the alpha blend mode, among ['Custom', 'Opaque', 'AlphaMask', 'Translucent'].

That's about all I managed to figure out based on Switch Toolbox and bmubin's codebases, but this was already very useful to get more accurate material imports in Blender by using Terrain textures, either when there is an entry in TextureMaps with the texture name "MaterialAlb" or when the mesh using the material has a vertex color available.

In case this PR never makes it in, a build with my fork can be downloaded here.