Improvements to Apple CgBI PNG handling

[Erik-White]

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Description

Following on from #3136 , this attempts to streamline and improve the related code as well as improve test coverage.

• CgBI images that are not supported are now explicitly rejected (i.e. bit depth != 8 or truecolor)
• Broke out ChunkedReadStream from ZlibInflateStream to separate the concerns instead of mixing modes in ZlibInflateStream
• Broke out CgBI transform from PngDecoderCore into new PngCgbiProcessor static class
• Added tests for PngCgbiProcessor
• Small improvements to the CgBI shuffle methods (e.g. construct the shuffle patterns only once, remove redundant floor(), replace magic numbers where possible)

[Erik-White]

Curious to hear your thoughts. The changes should all be relatively self-contained per commit in case there are sections that you don't think should be included.

[JimBobSquarePants]

This looks like a lot of changes. Will take time to review

[Erik-White]

Understood. As I mentioned in the description, each commit is fairly self contained but I could also break up the PR into a couple of smaller units if that would be helpful?

[JimBobSquarePants]

I'm not sure I'm comfortable with a type suffixed Stream that is no longer inherits Stream. This feels like we're violating runtime design guidelines.

[Erik-White]

My thinking was to use composition over inheritance here, especially since most the of the stream methods on ZlibInflateStream just throw NotSupportedException anyway.

But I'm sure ZlibInflateStream could just inherit from ChunkedReadStream (which inherits from Stream) if that would be preferrable? Or would it make sense to just rename this to something like ZlibInflateReader?

[JimBobSquarePants]

ZlibInflateReader is best IMO.

[JimBobSquarePants]

replace magic numbers where possible
What magic numbers?

[Erik-White]

replace magic numbers where possible
What magic numbers?

See this commit: e6f09d6
Just tried to use existing values where possible

[JimBobSquarePants]

Sorry for the radio silence here @Erik-White

I've had my head down on some complex drawing stuff. I'll give this one last pass now

[Copilot]

Pull request overview

This PR refactors and strengthens Apple CgBI PNG decoding in ImageSharp by separating concerns (chunked reads vs zlib header parsing, and CgBI pixel transforms vs decoding) and by adding explicit rejection + tests for unsupported CgBI variants.

Changes:

• Reject unsupported CgBI images (non–8-bit or non-truecolor) with a clear InvalidImageContentException.
• Extract CgBI scanline transform logic into a new PngCgbiProcessor with dedicated scalar/SIMD coverage tests.
• Extract chunk-boundary reading into ChunkedReadStream, simplifying ZlibInflateStream and enabling raw-DEFLATE handling for CgBI IDAT payloads.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/ImageSharp.Tests/TestImages.cs	Adds new synthetic CgBI fixtures for header-validation tests.
tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs	Adds Identify/Decode tests asserting incompatible CgBI headers are rejected.
tests/ImageSharp.Tests/Formats/Png/PngCgbiProcessorTests.cs	New tests validating SIMD/scalar parity for CgBI RGBA transform.
tests/Images/Input/Png/cgbi/colors-cgbi-palette.png	New LFS fixture for incompatible palette CgBI case.
tests/Images/Input/Png/cgbi/colors-cgbi-bitdepth16.png	New LFS fixture for incompatible 16-bit CgBI case.
src/ImageSharp/Formats/Png/PngDecoderCore.cs	Routes CgBI IDATs through raw DEFLATE + extracted processor; adds header validation hook.
src/ImageSharp/Formats/Png/PngCgbiProcessor.cs	New extracted CgBI scanline transform implementation (scalar + SIMD).
src/ImageSharp/Compression/Zlib/ZlibInflateStream.cs	Refactor: wrap zlib header parsing + deflate exposure over a chunked segment stream.
src/ImageSharp/Compression/Zlib/ChunkedReadStream.cs	New stream to read a sequence of length-delimited segments via a delegate.

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