NaNs are not showable

[johnnychen94]

Gray(NaN) and RGB(NaN) are not showable:

julia> Gray(NaN)
Error showing value of type Gray{Float64}:
ERROR: ArgumentError: element type N0f8 is an 8-bit type representing 256 values from 0.0 to 1.0,
  but the values (NaN, 1.0) do not lie within this range.
  See the READMEs for FixedPointNumbers and ColorTypes for more information.

The error message is a bit misleading, too.

ImageShow uses ImageCore.clamp01nan to handle this, I think we can perhaps move clamp01 and clamp01nan to Colors.

https://github.com/JuliaImages/ImageCore.jl/blob/a8f3d2c52070d9c166e1657ba25e72c8df00c742/src/map.jl#L3-L39

[kimikage]

At least Colors.jl does not implement the text display feature. So, this seems to be a side effect of other packages.

pkg> st --manifest
      Status `Manifest.toml`
  [3da002f7] ColorTypes v0.10.12
  [5ae59095] Colors v0.12.7
  [53c48c17] FixedPointNumbers v0.8.4
  [189a3867] Reexport v1.0.0
  [8f399da3] Libdl
  [37e2e46d] LinearAlgebra
  [9a3f8284] Random
  [9e88b42a] Serialization
  [2f01184e] SparseArrays
  [10745b16] Statistics

julia> using Colors

julia> Gray(NaN)
Gray{Float64}(NaN)

[johnnychen94]

Ah yes! Sorry I didn't describe it clearly; the error message I get shows when I use vscode.

[kimikage]

Colors, unlike ImageShow, displays a color or color arrays as "swatches" rather than an "image". So, it seems appropriate to return false in showable(). In fact, Jupyter implicitly fall back when an error occurs.