Specify color space (sRGB?)

I didn't see a mention of either "color space" nor "srgb" in https://github.com/google/iconvg/blob/main/spec/iconvg-spec.md, and neither did I find references in the original IVG spec. I believe the (implicit) color space should be specified, and because colors are limited to 32 bit, I suggest the common sRGB color space. As a corollary, conformant rasterizers must blend and compute gradients in linear RGB space.