mediawiki.mixins.less: Do not serve SVG to Opera 12 when fallback available

mediawiki.mixins.less: Do not serve SVG to Opera 12 when fallback available

We support it as a Grade A browser, which means that we should do our
best to provide the best possible experience. In my opinion, the best
possible experience involves not giving it any SVGs.

Opera 12 has issues when rendering SVG background-images together with
border-radius or background-size rules (see task T87504 for details
and examples), and both of these are becoming increasingly common in
our codebase.

Uses in CSS are not worth changing, as they are probably the simple
cases where it works correctly anyway.

Bug: T87504
Change-Id: Icdc2e1dcbcbc356042a064d703e1fe469004280b