Converting the LCMS2 transform to a 3D LUT early causes issues:
- It's a lossy process, the consumer will not be able to pick a
3D LUT size on their own.
- It requires unnecessary conversions and allocations: an intermediate
3D LUT is allocated, but the renderer already allocates one.
- It makes it harder to support arbitrary color transforms in the
renderer, because each type needs to be handled differently.
Instead, expose a function to evaluate a color transform, and use
that to build the 3D LUT in the renderer.
Color transform can have multiple types and these different types
want to store different metadata. We previously stored this metadata
directly on wlr_color_transform even for transforms that don't use it.
Instead, let's take the prior art from wlr_scene where each scene node
is built on a base node. Notice how wlr_color_transform_lut3d now has
a `struct wlr_color_transform base`. This is advantageous in multiple
ways:
1. We don't allocate memory for metadata that will never be used.
2. This is more type safe: Compositors can pass around a
struct wlr_color_transform_lut3d if they know they only want to use a
3d_lut.
3. This is more scalable. As we add more transform types, we don't have
to keep growing a monolithic struct.
