Fix animated png handling (issue #2708)

[SpaceCheetah]

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Description

Identified and fixed the issues mentioned in issue #2708.
Fix: ProcessInterlacedPaletteScanline now respects frameControl.XOffset
Fix: PngDecoderCore handling of PngDisposalMethod.RestoreToBackground and RestoreToPrevious, for specification compliance.
Metadata change: Added DefaultImageAnimated to PngMetadata. The apng format allows for the first frame to not be part of the animation sequence, which needs to be visible to users.
Fix: Decoding and encoding when the default image is not animated

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[JimBobSquarePants]

Note. We'll want to backport this to target the release/v3.1.x branch also.

[SpaceCheetah]

This can be backported to release/3.1.x with a simple cherry-pick. I can make a PR for that if you want.

[JimBobSquarePants]

This can be backported to release/3.1.x with a simple cherry-pick. I can make a PR for that if you want.

That would be amazing if you can!

[JimBobSquarePants]

Let's call this AnimateRootFrame. That matches ImageFrameCollection.RootFrame naming convention.

[JimBobSquarePants]

This looks great. Thanks for your perseverance.

If we can change the property name, then I will approve this for merging.

Thanks!