Remove vendor-prefixed background-clip CSS syntax
Summary:
`-moz-background-clip: padding` syntax was only supported until Firefox 3.6 according to https://web.archive.org/web/20130827100732/https://developer.mozilla.org/en-US/docs/Web/CSS/background-clip. Firefox 4.0 released 03/2011 supports standard `background-clip: padding-box`.
The same docs also imply that `-webkit-background-clip: padding-box` currently used by Phorge was never supported by browsers. Per https://caniuse.com/mdn-css_properties_background-clip_padding-box, Chrome 4 released 10/2010 supports standard `background-clip: padding-box`.
Compare current `webroot/rsrc/css/phui/phui-image-mask.css` which does not have this old vendor-prefixed syntax.
Test Plan:
* Open the Conpherence top bar dropdown, select "Persistent Chat", see that its `.jx-scrollbar-handle` based scrollbar still works as expected.
* Read CSS docs. Probably install ancient browser versions if you don't trust documentation. Try https://jigsaw.w3.org/css-validator/ and don't spot issues.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25970