Emissive and base colors are incorrectly converted

GLTF specifies that all colors in gltf json should be represented in linear space.

OBJ files have their colors defined in sRGB space.

`obj2gltf` now takes colors directly from OBJ and puts them in GLTF, but there should be conversion from sRGB to linear rgb colorspace.

Note that colors in textures are sRGB both in obj and gltf, only base color (diffuse) and emissive colors should be converted.

See this linked issue in Blender: https://projects.blender.org/blender/blender-addons/issues/102905#issuecomment-11438

and the corresponding discussion in gltf spec repository: https://github.com/KhronosGroup/glTF/issues/1609#issuecomment-491097601

The corresponding locations in code where the change should be made:

https://github.com/CesiumGS/obj2gltf/blob/ce0c36e28193205f6465a22c03f554fe22597f46/lib/loadMtl.js#L139-L144

https://github.com/CesiumGS/obj2gltf/blob/ce0c36e28193205f6465a22c03f554fe22597f46/lib/loadMtl.js#L131-L136

EDIT:

Let me also link Bevy engine PR that also fixed this: https://github.com/bevyengine/bevy/pull/6828