Improve `compN` for non-parametric `ColorAlpha` types

[kimikage]

Since only AGray32 and ARGB32 are defined as non-parametric TransparentColor types, I did not notice this problem, but it would be better to specialize compN for ColorAlpha as well as AlphaColor.

This is not an urgent issue, since it can be solved by overloading compN by users or packages which define custom color types. Therefore, I will make this a part of v0.12 instead of v0.11, even though this is a non-breaking change as long as alpha(c) works.

[codecov[bot]]

Codecov Report

Merging #245 (1cbe7a7) into master (8fc7034) will increase coverage by 0.02%. The diff coverage is 100.00%.

:exclamation: Current head 1cbe7a7 differs from pull request most recent head eedc2d3. Consider uploading reports for the commit eedc2d3 to get more accurate results

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   82.25%   82.28%   +0.02%     
==========================================
  Files           8        8              
  Lines         727      728       +1     
==========================================
+ Hits          598      599       +1     
  Misses        129      129              

Impacted Files	Coverage Δ
src/traits.jl	98.83% <100.00%> (+<0.01%)	:arrow_up:

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[kimikage]

It's been half a month since the release of v0.11.0, so I'd like to start the v0.12 series.