Add support for KHR_materials_specular

[romainguy]

The KHR_materials_specular extension is fairly simple and exposes three new parameters:

• specularFactor, float, default = 1.0f
• specularColorFactor, float3, default = {1.0f, 1.0f, 1.0f}
• specularTexture, a 4-channel texture that combines specularFactor and specularColorFactor

This change has been tested by test_write using a Khronos sample model.

[prideout]

Looks good. For reference, the spec is here and is a part of the "PBR Next" standardization effort.

I hope people won't confuse this with the specular-glossiness extension. The naming scheme that Romain used in the code is consistent with the Khronos spec, so I think it's fine.

[romainguy]

The extension is called materials_specular, but I followed the existing convention in cgltf code to elide the materials_ prefix. In general it might be better to stick to the extensions names more closely, if only to make it easier for API users to find what they are looking for. Also, extensions are starting to proliferate and there's a question of whether they should be handled differently in cgltf.

[romainguy]

Should I do anything else on this PR?

[jkuhlmann]

No, no, no, sorry, all good! 😄

[romainguy]

No need to apologize, I appreciate you taking the time to take a look. Thank you!

[jkuhlmann]

I kind of wanted to write up my thoughts on a new approach to extensions first and kept this open as a reminder... Anyway, feedback on issue #125 would be appreciated.