Fix gaussian blurring changing the blur colour when transperancy is used

[Turnerj]

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Description

Firstly, all credit goes to @JimBobSquarePants for identifying the fix.

Blurring an image that has a transparent background can lead to strange results depending on the colour.
For example, take this code:

using var image = new Image<Rgba32>(400, 400);
image.Mutate(x => x.Fill(Color.ParseHex("48f25b"), new Rectangle(100, 100, 200, 200)).GaussianBlur(20));

Master generates this:

This PR generates:

I do want to add tests for it, just working out how to do it like how you have your Bokeh blur tests.

[codecov]

Codecov Report

Merging #1834 (088e92c) into master (31e6230) will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #1834    +/-   ##
=======================================
- Coverage      87%     87%    -1%     
=======================================
  Files         937     937            
  Lines       48420   48421     +1     
  Branches     6054    6054            
=======================================
- Hits        42323   42222   -101     
- Misses       5095    5193    +98     
- Partials     1002    1006     +4     

Flag	Coverage Δ
unittests	87% <100%> (-1%)	⬇️

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Impacted Files	Coverage Δ
...s/Convolution/Convolution2PassProcessor{TPixel}.cs	61% <100%> (+<1%)	⬆️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	87% <0%> (-11%)	⬇️
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	89% <0%> (-9%)	⬇️

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[Turnerj]

Took a little while but I worked out how to do the reference images for tests (and yep, I did run the optimizer on them so the file size is smaller for the repo).

This is the source image:

Running the test on master:

Running the test on this PR:

As you can see, the blurring was doing all sorts of weirdness with colours without that line.

[JimBobSquarePants]

[antonfirsov]

This can be made conditional on PixelTypeInfo.AlphaRepresentation:

ImageSharp/src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeProcessor{TPixel}.cs

Lines 191 to 194 in 9777a3b

	PixelAlphaRepresentation? alphaRepresentation = PixelOperations<TPixel>.Instance.GetPixelTypeInfo()?.AlphaRepresentation;
	// Premultiply only if alpha representation is unknown or Unassociated:
	bool needsPremultiplication = alphaRepresentation == null || alphaRepresentation.Value == PixelAlphaRepresentation.Unassociated;

[JimBobSquarePants]

Yeah. Something to look into. I keep meaning to see what else we can add to PixelTypeInfo actually that would help us in other places.