add colorspace metadata to surfaces and items, and use it in the OpenGL renderer

A small preparation step for implementing the color management protocol(s).

Note that with this commit, the color management shader (but not the whole color management pipeline) will always be run. It shouldn't add a lot of instructions to the shader, so I think it shouldn't impact performance in a noticeable way on any hardware. For the case that I'm wrong about that, I've made this as a separate MR, so that we can notice that before we merge more color management bits that depend on this.