Implement discontiguous buffer handling

[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

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

[codecov]

Codecov Report

Merging #1109 into master will increase coverage by 0.1%.
The diff coverage is 79.31%.

@@            Coverage Diff            @@
##           master    #1109     +/-   ##
=========================================
+ Coverage   82.04%   82.15%   +0.1%     
=========================================
  Files         680      686      +6     
  Lines       28562    29048    +486     
  Branches     3238     3308     +70     
=========================================
+ Hits        23434    23864    +430     
- Misses       4453     4485     +32     
- Partials      675      699     +24

Flag	Coverage Δ
#unittests	82.15% <79.31%> (+0.1%)	⬆️

Impacted Files	Coverage Δ
...ry/DiscontiguousBuffers/MemoryGroup{T}.Consumed.cs	90.9% <ø> (ø)
src/ImageSharp/Advanced/AdvancedImageExtensions.cs	70% <0%> (-10%)	⬇️
src/ImageSharp/Image.Decode.cs	96.77% <100%> (+0.1%)	⬆️
src/ImageSharp/Memory/BufferArea{T}.cs	96.77% <100%> (+0.47%)	⬆️
src/ImageSharp/Memory/Buffer2D{T}.cs	97.72% <100%> (-2.28%)	⬇️
...emory/DiscontiguousBuffers/MemoryGroup{T}.Owned.cs	95% <100%> (ø)
src/ImageSharp/Image.LoadPixelData.cs	100% <100%> (ø)	⬆️
src/ImageSharp/ImageFrame.LoadPixelData.cs	85.71% <100%> (+5.71%)	⬆️
src/ImageSharp/ImageFrameCollection{TPixel}.cs	89.28% <100%> (+0.19%)	⬆️
src/ImageSharp/Formats/Jpeg/Components/Block8x8.cs	52.56% <100%> (ø)	⬆️
... and 50 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a3f169f...ad2632f. Read the comment docs.

[antonfirsov]

Looks like I managed to fix everything, removed the WIP tag, ready for final review.

[JimBobSquarePants]

Fantastic work!

All looks good to me, just spotted a possible optimization opportunity in RowOctet<T>

I like that we can implement any enhancements to individual codecs in the future.