Bump ChunkedMemoryStream buffer length

[JimBobSquarePants]

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Description

Increase ChunkedMemoryStream chunk length to 128k inline with RecycledMemoryStream

[antonfirsov]

LGTM (DefaultBufferLength change)

[codecov]

Codecov Report

Merging #1328 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1328   +/-   ##
=======================================
  Coverage   82.74%   82.75%           
=======================================
  Files         689      689           
  Lines       30933    30938    +5     
  Branches     3508     3508           
=======================================
+ Hits        25597    25602    +5     
  Misses       4617     4617           
  Partials      719      719           

Flag	Coverage Δ
#unittests	82.75% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/ImageSharp/IO/ChunkedMemoryStream.cs	81.11% <100.00%> (+0.41%)	⬆️

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