The axial/radial ramps and the function-based lattice converted one color per
call, i.e. one Wasm round-trip each for `IccColorSpace` (hence for
`/DeviceCMYK`). Add `ColorSpace.getRgbItems`, overridden by `IccColorSpace` and
`AlternateCS`, to convert a whole batch at once.
It fixes#5046.
We just generate a mesh for the pattern rectangle where the color of each vertex is computed from the function.
Since the mesh is generated in the worker we don't really take into account the current transform when it's drawn.
That being said, there are maybe some possible improvements in using directly the gpu for the shading creation
which could then take into account the current transform, but it could only work with ps function we can convert
ino wgsl language and simple enough color spaces (gray and rgb).
