Add support for Decoding BI_ALPHABITFIELDS

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

This PR add the support for decoding bitmaps with the compression type: BI_ALPHABITFIELDS.
This is a special kind of bitmap type which was introduced with windows CE: https://docs.microsoft.com/en-us/previous-versions/windows/embedded/aa452885(v%3Dmsdn.10)

This also fixes an issue with a rare edge case, decoding bitmaps with a less than full sized palette.

[codecov]

Codecov Report

Merging #832 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #832      +/-   ##
==========================================
+ Coverage   88.84%   88.84%   +<.01%     
==========================================
  Files        1015     1015              
  Lines       44148    44167      +19     
  Branches     3180     3181       +1     
==========================================
+ Hits        39222    39241      +19     
  Misses       4201     4201              
  Partials      725      725

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs	99.01% <100%> (+0.09%)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	83.59% <100%> (+0.37%)	⬆️

---

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[codecov]

Codecov Report

Merging #832 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #832      +/-   ##
==========================================
+ Coverage   88.84%   88.84%   +<.01%     
==========================================
  Files        1015     1015              
  Lines       44148    44167      +19     
  Branches     3180     3181       +1     
==========================================
+ Hits        39222    39241      +19     
  Misses       4201     4201              
  Partials      725      725

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs	99.01% <100%> (+0.09%)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	83.59% <100%> (+0.37%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13ff434...1208628. Read the comment docs.

[JimBobSquarePants]

@brianpopow Just so you know you are now known as the Bitmap King 👑