Improvements to Apple CgBI PNG handling

[Erik-White]

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 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

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?