Auto repair Png options to use Bit8.

[JimBobSquarePants]

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Description

Fixes #935

Rather than throwing when ColorType/BitDepth options are passed that are not supported by the PNG specification we can use PngBitDepth.Bit8 to fix up the options.

This bit depth is the only one supported by all color type values.

[codecov]

Codecov Report

Merging #1395 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
+ Coverage   82.87%   82.88%   +0.01%     
==========================================
  Files         690      690              
  Lines       30903    30903              
  Branches     3545     3544       -1     
==========================================
+ Hits        25610    25614       +4     
+ Misses       4572     4570       -2     
+ Partials      721      719       -2     

Flag	Coverage Δ
#unittests	82.88% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
...ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs	92.40% <100.00%> (+5.06%)	⬆️

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[brianpopow]

looks good