I want to add support for PBR materials extension to OBJ, but the way
current I/O code syntax handling was done made it quite cumbersome
to extend the number of MTL textures/parameters.
Simplify all that by removing FormatHandler template on "syntax"
that gets routed through keyword enums, and instead just have
simple `write_obj_*` and `write_mtl_*` functions.
Simplify MTLMaterial to not contain a map of textures (that is always
fully filled with all possible textures), instead now there's
a simple array. Rename `tex_map_XX` to `MTLTexMap`.
All this does not affect behavior or performance, but it does result
in 170 fewer lines of code, and saves a couple kilobytes of executable
size.
68f234b8ab