Unified jpeg benchmark

[br3aker]

Prerequisites

• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

This PR adds 'unified' jpeg decoder benchmark with different kinds of jpeg images for somewhat baseline performance comparison of new code/optimizations. I've mainly used already existing images for this benchmark it by no means has all of different jpeg kinds - no progressive 4:4:4 for example. Benchmark image collection should naturally grow over time.

I've also restructured formats behcmarks folder to this instead of a one big folder of all formats benchmarks:

[codecov]

Codecov Report

Merging #1844 (e143093) into master (413c90c) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1844   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         937     937           
  Lines       48420   48420           
  Branches     6054    6054           
======================================
+ Hits        42324   42325    +1     
+ Misses       5094    5093    -1     
  Partials     1002    1002           

Flag	Coverage Δ
unittests	87% <ø> (+<1%)	⬆️

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Impacted Files	Coverage Δ
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	97% <0%> (-1%)	⬇️
...mageSharp/Formats/Webp/Lossless/NearLosslessEnc.cs	100% <0%> (+3%)	⬆️

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[JimBobSquarePants]

Good idea 👍