This reduces the payload of CSS slightly (and in the few cases where these
were not just present but @embed'ed, significantly), at a cost of giving
those user agents which don't support the un-prefixed rule the icons as
PNGs rather than SVGs (i.e. Chrome < 25, Safari < 6.1, Android < 4.4).
We made this change for MediaWiki back in November 2015 in T118815. This
repo is not alone in not having noticed. :-)
Change-Id: I1469bce3dd6c6eed0d291a0cb8a19eceeaff4c10
f47692a390