PHUIImageMaskView: Replace checkerboard image with repeating-conic-gradient CSS
Summary:
Note that this is a trade-off between improved Dark Mode and browser compatibility: Such CSS is only supported since Chrome 69/Opera 56 (09/2018), Firefox 83 (11/2020), Safari 12.1 (03/2019), and Edge 79 (01/2020) according to https://caniuse.com/?search=repeating-conic-gradient. Older browsers will display the `{$page.content}` color instead with no checkerboard pattern.
PHUIImageMaskView seems to only be used to create thumbs in a Pholio context, in `PholioTransactionView` and `PholioInlineController`.
See similar rP03e8c8ab0cdb.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to http://phorge.localhost/uiexample/view/PHUIImageMaskExample/
* Optionally, check things in Pholio but I am too lazy for that
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26425