StackOverflowError when attempting undefined color space conversion

[kimikage]

Although undefined color space conversion should result in an error, the error message is not helpful.

julia> using ColorTypes

julia> struct MyColor{T} <: Color{T, 3}; c1::T; c2::T; c3::T; end

julia> convert(RGB, MyColor{Float32}(1, 2, 3)) # without Colors.jl
ERROR: No conversion of MyColor{Float32}(1.0f0,2.0f0,3.0f0) to RGB{Float32} has been defined

julia> using Colors

julia> convert(RGB, MyColor{Float32}(1, 2, 3)) # with Colors.jl
ERROR: StackOverflowError:

A fix for this issue could theoretically affect downstream packages. However, given that there have been no issue reports on this so far, I think it can be included in the v0.12 series as a bug fix in practice.

[kimikage]

This is due to the circular calls between the conversion to RGB and the conversion to XYZ.

From the viewpoint of color management, I think the conversion to XYZ should be implemented preferentially over the conversion to RGB, but in actual cases where the stack overflow is a problem, the conversion to RGB seems to be defined preferentially.

Also, due to historical reasons, we currently limit the conversion source to Color3, but I think this should be more generalized.