Add ANI decoder support

[Poker-sang]

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

implements AniDecoder for ImageSharp, and encoder if the pr approved

Comment

Since the ANI file may contain a two-dimensional array of ImageFrames, I flattened the original ImageFrames as follows

I have kept all Metadata as much as possible, but this may result in a structure that is not intuitive, and may require further discussion with you) reserved the possibility of future modifications

ANI is also a RIFF file, so I referenced RiffHelper, perhaps we should move it out of the WEBP namespace

[1]
[2]

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ Poker-sang
❌ zxbmmmmmmmmm
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[JimBobSquarePants]

Thanks @Poker-sang I’ll have a deep look at this asap.