gltfpack: Support UV transforms when converting .obj/.mtl files

[kzhsw]

The mtl spec (not sure if it's official): https://paulbourke.net/dataformats/mtl/
Test file (taken from here): cube_with_materials.zip

This map_Kd -o 0.1 0.2 0.0 -s 2.0 3.0 0.0 texture.png, after loaded to gltfpack and exported to gltf, is gone.
Not sure if this belongs to fast_obj or here.

Expected:

Actual:

[zeux]

fast_obj doesn't seem to support this, so this is not really actionable until it does.

[kzhsw]

Ok, I filed an issue in fast_obj here: thisistherk/fast_obj#56