Implement discontiguous buffer handling - #1109

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Description

Marked as WIP because processors and codecs need adaption, but the core logic is ready to review!

• MemorySource<T> has been replaced with MemoryGroup<T>, which is a list of uniformly sized buffers (IMemoryOwner<T> and Memory<T>)
• Several helpers intend to provide span-like functionalities in MemoryGroupExtensions eg. .CopyTo()
• MemoryGroupView<T> is an indirection to expose these memory buffers in a safe way, see docs and comments for more info
• The methods exposed over AdvancedMemoryExtensions have been changed to reflect the new memory model, .GetPixelSpan<T>() has been made obsolete
• buffer2d.GetSpan<T>() ---> buffer2d.GetSingleSpan<T>(), temporarily marked [Obsolete] to guide refactor

TODO

We should no longer process Buffer2D as a single span (except of a few special cases like resize KernelMap). Guidance: go through all [Obsolote] warnings.

• Add tests for each codec and processor in a similar manner like it has been added to Jpeg tests. (Artifically limit buffer capacity of allocator to emulate the discontiguous case.)
• Fix remaining codecs
• Fix remaining processors

Resolves #898, Fixes #805