cache max in ConvertToRgba

[SimonCropp]

not sure if this is a fast path?

[codecov]

Codecov Report

Merging #982 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #982      +/-   ##
==========================================
- Coverage    89.7%   89.59%   -0.12%     
==========================================
  Files        1097     1097              
  Lines       48556    48544      -12     
  Branches     3422     3422              
==========================================
- Hits        43558    43492      -66     
- Misses       4295     4300       +5     
- Partials      703      752      +49

Impacted Files	Coverage Δ
...der/ColorConverters/JpegColorConverter.FromCmyk.cs	100% <100%> (ø)	⬆️
...oder/ColorConverters/JpegColorConverter.FromRgb.cs	100% <100%> (ø)	⬆️
...olorConverters/JpegColorConverter.FromGrayScale.cs	100% <100%> (ø)	⬆️
...der/ColorConverters/JpegColorConverter.FromYccK.cs	100% <100%> (ø)	⬆️
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.9% <0%> (-10.1%)	⬇️
...olorSpaces/Conversion/ColorSpaceConverter.Adapt.cs	60% <0%> (-8.58%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	87.64% <0%> (-6.75%)	⬇️
src/ImageSharp/Primitives/ValueSize.cs	78.94% <0%> (-5.27%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	82.05% <0%> (-5.13%)	⬇️
....Tests/TestUtilities/Tests/TestEnvironmentTests.cs	63.63% <0%> (-4.55%)	⬇️
... and 22 more

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