Add loading support for .cube LUT format

[mvaligursky]

We support LUT color postprocssing (added here). This works great, but as the linear LUT is stored in 8bit png (and not in gamma space), this adds some banding in some cases.

Ideally we support loading of .cube files directly. Internally, they typically store values with higher precision (floats), and we should store them in RGBA16F textures or equivalent.

Alternatively, we could consider using .hdr format to store them, but the pipeline for users might be more complex - we'd need to figure it out.