Gif. Use Buffer2D throughout.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Refactors the IndexedImageFrame<TPixel> to use Buffer2D<byte> internally and the gif decoder/encoder to allow decoding/encoding of large images.

[codecov]

Codecov Report

Merging #1158 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1158   +/-   ##
=======================================
  Coverage   82.26%   82.27%           
=======================================
  Files         681      681           
  Lines       29331    29347   +16     
  Branches     3295     3297    +2     
=======================================
+ Hits        24129    24145   +16     
  Misses       4514     4514           
  Partials      688      688           

Flag	Coverage Δ
#unittests	82.27% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...ageSharp/Common/Extensions/ComparableExtensions.cs	64.00% <ø> (ø)
src/ImageSharp/Formats/Gif/GifDecoderCore.cs	83.76% <100.00%> (ø)
src/ImageSharp/Formats/Gif/GifEncoderCore.cs	94.91% <100.00%> (ø)
src/ImageSharp/Formats/Gif/LzwDecoder.cs	95.78% <100.00%> (+0.49%)	⬆️
src/ImageSharp/Formats/Gif/LzwEncoder.cs	98.33% <100.00%> (+0.13%)	⬆️
src/ImageSharp/IndexedImageFrame{TPixel}.cs	100.00% <100.00%> (ø)
...ocessors/Quantization/QuantizeProcessor{TPixel}.cs	100.00% <100.00%> (ø)

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