Loaders: Unpack colors to working color space

[donmccurdy]

Several loaders were converting model input colors to explicitly Linear-sRGB, with methods like convertSRGBToLinear. For future compatibility with wide gamut and HDR workflows, it would be better to target the working color space (Linear-sRGB by default, but may be changed by the user) instead. With this PR, I change the remaining affected loaders to these patterns:

// tells Color.js that the source color was sRGB, causing
// implicit conversion to working color space
color.setRGB( r, g, b, SRGBColorSpace );

// explicitly converts from the given source color space to
// the working color space
ColorManagement.toWorkingColorSpace( color, SRGBColorSpace );