Imrpoved Gray8 accuracy

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Adding 0.5f in Get8BitBT709Luminance() to get a rounded result
  • This makes the Gray8 -> Rgba32 -> Gray8 conversion return the original value to itself for all inputs (31 was an outlier for example)
  • Please let me know if I misunderstood how should this work, but for me it seems much reasonable with the rounding!
• More efficient & more accurate FromVector4 implementation
• More tests

[JimBobSquarePants]

That makes sense me. We're not losing when rounding then.

[codecov]

Codecov Report

Merging #754 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #754      +/-   ##
==========================================
+ Coverage    88.4%   88.41%   +0.01%     
==========================================
  Files         997      996       -1     
  Lines       42325    42385      +60     
  Branches     3142     3142              
==========================================
+ Hits        37416    37476      +60     
  Misses       4225     4225              
  Partials      684      684

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/PixelFormats/Gray8Tests.cs	100% <100%> (ø)	⬆️
src/ImageSharp/Common/Helpers/ImageMaths.cs	87.17% <100%> (ø)	⬆️
...geSharp/PixelFormats/PixelImplementations/Gray8.cs	79.16% <100%> (+1.38%)	⬆️
tests/ImageSharp.Tests/TestImages.cs	100% <0%> (ø)	⬆️
...Sharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs	100% <0%> (ø)	⬆️
.../ImageSharp.Tests/Common/EncoderExtensionsTests.cs

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[antonfirsov]

@JimBobSquarePants should we merge this then?

[JimBobSquarePants]

Yeah, I was just waiting on it to finish building