Expose supported capabilities for colorspace and tonemapping

[kainino0x]

For now in #362 and #363 we are adding the ability to request color spaces and tonemap modes but we don't have any way to see the supported capabilities.

Previously discussed in #200, see there for discussion.
We probably don't need to do this for v1 but it could be nice if we know what to do.

[Kangz]

Overall it seems that we are trying to add core API here, without knowing exactly how we're going to expose it outside of the Web. For that reason I think it would be a better fit to make this an Emscripten extension and not part of the core API (at least until we know what to do here).