Single row value delegate option

[Sergio0694]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

This PR reintroduces the single-row value delegates for pixel processors. It includes two new APIs for the ParallelHelper class, so that individual processor can choose whether to iterate over single rows or over row intervals, depending on their needs, so that it's possible to find the best possible balance between performance and code clarity.

Opening this not as a draft PR just to have the CI run on it, but this is NOT READY TO MERGE.

[Sergio0694]

@antonfirsov Mmmh, getting a very nice System.AccessViolationException on .NET472, that's weird considering that the tests seem to be running fine on .NET Core 3.1 and 2.1 🤔

EDIT: all tests are passing just fine locally, I've bumped the CI.

[antonfirsov]

AccessViolationException worries me, might indicate a sporadic memory corruption bug. Do we have any data where did it happen?

Can you run stress it a bit locally against net472? A PS script like this might do the job:

for($i = 0; $i -lt 5; $i++)
{
    dotnet xunit -c Release --no-build -f net472
}

for($i = 0; $i -lt 5; $i++)
{
    dotnet xunit -c Release --no-build -f net472 -x86
}

[JimBobSquarePants]

@Sergio0694 What is the stack trace for that exception? I have a rich history of chasing them down in the jpeg decoder so I may prove useful.

[Sergio0694]

@JimBobSquarePants Ah crap, it was somewhere in the ResizeProcessor<TPixel> tests I think, but I restarted the CI yesterday and the logs are gone, is there a way to get them back somehow? 😟

[JimBobSquarePants]

@Sergio0694 I had a look through the list of actions and I can't find anything other than the CoreFlags issues which have been fixed.

https://github.com/SixLabors/ImageSharp/actions/runs/46311348

I've also thoroughly reviewed the code and there are no changes that you have made that could cause the introduction of an AccessViolationException. I'm approving this.

[Sergio0694]

@JimBobSquarePants Sounds great, awesome! 😄 🚀
Also glad to hear I didn't accidentally mess anything up then ahahahah