Fix Png Encoder Compression

[JimBobSquarePants]

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Description

Replaces the .NET DeflateStream implementation used by the png encoder with a fully managed one ported from SharpZipLib and heavily modified.

This fixes #1027 with a 6x improvement in compression with the supplied image.

Benchmarks are.... ok I guess.... I managed to make the ported DeflaterOuputStream and accompanying classes around 40% faster but we've lost a fair amount of performance compared to the native SIMD accelerated version used by .NET Core. I think the massive compression gains are worth it though but if anyone fancies a crack at further optimization please give it a go.

Before

Method	Job	Runtime	LargeImage	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
'System.Drawing Png'	Clr	Clr	False	16.00 ms	1.429 ms	0.0783 ms	1.00	0.00	500.0000	218.7500	218.7500	2.03 MB
'ImageSharp Png'	Clr	Clr	False	31.80 ms	2.094 ms	0.1148 ms	1.99	0.01	500.0000	500.0000	500.0000	2.25 MB
'System.Drawing Png'	Core	Core	False	16.85 ms	10.899 ms	0.5974 ms	1.00	0.00	500.0000	218.7500	218.7500	2.03 MB
'ImageSharp Png'	Core	Core	False	21.19 ms	7.654 ms	0.4195 ms	1.26	0.07	562.5000	531.2500	531.2500	2.24 MB

After

Method	Job	Runtime	LargeImage	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
'System.Drawing Png'	Clr	Clr	False	18.15 ms	4.199 ms	0.2302 ms	1.00	0.00	500.0000	218.7500	218.7500	2.03 MB
'ImageSharp Png'	Clr	Clr	False	43.28 ms	1.944 ms	0.1066 ms	2.38	0.03	416.6667	416.6667	416.6667	2.3 MB
'System.Drawing Png'	Core	Core	False	18.33 ms	2.518 ms	0.1380 ms	1.00	0.00	500.0000	218.7500	218.7500	2.03 MB
'ImageSharp Png'	Core	Core	False	43.05 ms	5.185 ms	0.2842 ms	2.35	0.03	416.6667	416.6667	416.6667	2.3 MB

[codecov]

Codecov Report

Merging #1054 into master will decrease coverage by 0.03%.
The diff coverage is 86.77%.

@@            Coverage Diff            @@
##           master   #1054      +/-   ##
=========================================
- Coverage   89.93%   89.9%   -0.04%     
=========================================
  Files        1124    1131       +7     
  Lines       49772   50654     +882     
  Branches     3513    3717     +204     
=========================================
+ Hits        44764   45539     +775     
- Misses       4287    4357      +70     
- Partials      721     758      +37

Impacted Files	Coverage Δ
.../ImageSharp/Formats/Png/Zlib/DeflateThrowHelper.cs	0% <0%> (ø)
src/ImageSharp/Formats/Png/Zlib/Crc32.cs	92.75% <100%> (+0.1%)	⬆️
src/ImageSharp/Formats/Png/Zlib/Adler32.cs	79.31% <100%> (+0.73%)	⬆️
...ts/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs	99.51% <100%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngEncoderCore.cs	96.91% <100%> (ø)	⬆️
...c/ImageSharp/Formats/Png/Zlib/DeflaterConstants.cs	100% <100%> (ø)
...c/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs	66.66% <46.66%> (+3.5%)	⬆️
...mageSharp/Formats/Png/Zlib/DeflaterOutputStream.cs	58% <58%> (ø)
src/ImageSharp/Formats/Png/Zlib/Deflater.cs	58.62% <58.62%> (ø)
src/ImageSharp/Formats/Png/Zlib/DeflaterEngine.cs	86.62% <86.62%> (ø)
... and 9 more

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[JimBobSquarePants]

This appears to be a hotspot according to dotTrace.

[JimBobSquarePants]

I've done my best. Get stuck in please.

[JimBobSquarePants]

I think I'm going to go ahead and merge this. It isn't slow and we get good compression. Any other gains we can make down the line.