Add a gfx::ColorSpace to SolidColorDrawQuad. All other draw quads get their color space from their resource, but solid color draw quads need to specify one on the quad.

All SolidColorDrawQuads found in their natural habitat will have an sRGB, so this isn't necessary functionality.

But, this will be the mechanism through which we'll be writing pixel tests to ensure that our ColorTransform shaders match the CPU implementation.