Runtime texture color space settings

[stevenvergenz]

glTF's PBR material definition specifies the color spaces that its various textures should be in: sRGB for baseColor, linear for metallicTexture, etc. However, Unity's DownloadHandlerTexture class used by UnityWebRequest always assumes the texture is sRGB, and does not provide an option to specify. So an alternative loader/handler will probably need to be created to properly import these textures.

^ @tparisi