alphaMode is now converted from a std::string to an aiString and back to std::string, since aiString is easier to store and retrieve from aiMaterial properties than std::string
Fixes issues of alphaMode being written out as `\fOPAQUE\0\0\0\0\0\0…`
Previously, only one primitive was supported, in fact memory was corrupted
when more than one primitive was found per glTF mesh.
In this change, each primitive is unrolled as a new Assimp Mesh, resulting
in multiple Assimp meshes per node when multiple primitives exist per
glTF mesh. This is required in the general case, since glTF primitives can
have different material bindings and primitive modes.
Samplers are now imported into assimp internal format from textures;
Samplers have defaults as per spec;
Sampler enums are strongly typed with UNSET values;
Samplers are exported properly;
Sampler filters are exported as well;
Samplers are re-used across textures on export
Default sampler values are not written
pbrMetallicRoughness and pbrSpecularGlossiness as structs; persist textureinfo properties from start to finish; persist pbrSpecularGlossiness (via extensionsUsed) usage from start to finish
