Web-stories-wp: AMP Sanitization breaks gradients (again)

[swissspidy]

Bug Description

This is essentially #4529, but now the other way around:

When _not_ using the AMP plugin, gradients will be stripped on the frontend.

The reason is a bug in PHP-CSS-Parser, which the AMP plugin solves by patching the package.

Since we cannot just let the AMP plugin deal with the patches because of https://github.com/cweagans/composer-patches/issues/315, we copied all the patches to our repo.

However, the patches section was accidentally omitted.

But even with that set I'm currently getting issues with patches not applying cleanly.

So this needs some investigation.

Expected Behaviour

PHP-CSS-Parser patches should be applied cleanly.

Steps to Reproduce

1. Apply gradient to element
2. Notice that gradient disappears on frontend.

Workaround: install AMP plugin.

Screenshots

Additional Context

• Plugin Version: 1.1.0
• Operating System: macOS
• Browser: Chrome 86

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Acceptance Criteria

Implementation Brief

[csossi]

Verified in QA

FE - radial and liner:

[o-fernandez]

I see beautiful gradients!