Use MemoryAllocator in FixedCapacityPooledMemoryStream

[antonfirsov]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

Integrating FixedCapacityPooledMemoryStream with MemoryAllocator and moving duplicate async decoder code to a utility class.

ArrayPool.Shared is good when (1) the rented buffer is relatively small (2) the Rent operation is a bottleneck on a hot path. In image processing these conditions are barely true. For a byte array > 1MB, the shared pool is always allocating a new array, which is a common for raw image streams.

[codecov]

Codecov Report

Merging #1223 into master will increase coverage by 0.14%.
The diff coverage is 79.03%.

@@            Coverage Diff             @@
##           master    #1223      +/-   ##
==========================================
+ Coverage   82.40%   82.54%   +0.14%     
==========================================
  Files         696      697       +1     
  Lines       30553    30506      -47     
  Branches     3482     3467      -15     
==========================================
+ Hits        25176    25181       +5     
+ Misses       4658     4606      -52     
  Partials      719      719              

Flag	Coverage Δ
#unittests	82.54% <79.03%> (+0.14%)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Formats/ImageDecoderUtilities.cs	28.57% <28.57%> (ø)
src/ImageSharp/Image.FromStream.cs	76.56% <50.00%> (ø)
src/ImageSharp/Formats/Gif/GifDecoderCore.cs	83.96% <85.71%> (+4.76%)	⬆️
src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs	92.89% <88.88%> (+3.21%)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	94.45% <100.00%> (+0.91%)	⬆️
src/ImageSharp/Formats/Png/PngDecoderCore.cs	90.63% <100.00%> (+2.53%)	⬆️
src/ImageSharp/Formats/Tga/TgaDecoderCore.cs	91.29% <100.00%> (+3.71%)	⬆️
...c/ImageSharp/IO/FixedCapacityPooledMemoryStream.cs	100.00% <100.00%> (ø)
src/ImageSharp/Memory/MemoryAllocatorExtensions.cs	100.00% <100.00%> (ø)

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[JimBobSquarePants]

Beautiful!