Fix for #576, using explicit struct layout for bgr24 and rgb24

[brianpopow]

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Description

This PR addresses to fix the issue that ImageCloneTests.CloneAs_ToBgr24() was failing sporadically.
rgb24 struct was also changed, because it may also be affected by this issue.

[codecov]

Codecov Report

Merging #591 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #591      +/-   ##
==========================================
+ Coverage   88.73%   88.78%   +0.04%     
==========================================
  Files         854      854              
  Lines       36172    36172              
  Branches     2620     2620              
==========================================
+ Hits        32099    32114      +15     
+ Misses       3290     3275      -15     
  Partials      783      783

Impacted Files	Coverage Δ
src/ImageSharp/PixelFormats/Bgr24.cs	80% <ø> (+4%)	⬆️
src/ImageSharp/PixelFormats/Rgb24.cs	77.55% <ø> (ø)	⬆️
tests/ImageSharp.Tests/Image/ImageCloneTests.cs	100% <ø> (+24.07%)	⬆️

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[antonfirsov]

Unksipped CloneAs_ToBgr24. We should merge this, if the build succeeds,