V2 backport: 2243 - Ignore unknown chunks for WEBP

[sonbua]

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Description

Add backported fix for #2243
Relates to #2245

[CLAassistant]

All committers have signed the CLA.

[sonbua]

Would someone help to review @JimBobSquarePants @brianpopow? This basically backports PR #2245, ignore unknown chunks for WEBP, to v2.

[JimBobSquarePants]

We don't normally backport fixes into legacy versions unless there are specific security reasons. Users are encouraged to upgrade to the latest released version.

[sonbua]

This could be closed then.

[a-stankevich]

We have to stick with v2 thanks to its support of older .net versions. And v2 keeps crashing more and more these days on webp files because of this header issue.

Any chance to reconsider this issue for backporting to v2 for those who can't afford upgrading to v3 yet?

Thank you