Mono AOT decoder workaround for slow jpeg decoding.

[JimBobSquarePants]

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Description

This PR is an attempt to work around issues found with the Mono AOT compiler which causes slow performance on IOS Android

dotnet/runtime#71210

In the linked Issue, Analysis from the Mono team highlighted this indirection as a culprit.

The AOT compiler does appear to have problems figuring out which instances to generate. In this specific case, the caller is
ImageDecoderUtilities:Decode<Rgba32> which calls IImageDecoderInternals::Decode on an argument. So in theory, the aot compiler could figure out that the call could possible go to JpegDecoderCore::Decode<Rgba32> and generate that instance. Currently, this kind of analysis is not done.

This PR removes that indirection completely by introducing an internal base class, ImageDecoderCore for all XXDecoderCore instances. In addition, seeding has been introduced for SpectralConverter<TPixel> and others.

This is currently untested but I'm confident that this should improve matters.