Make transform a mapped attribute for SVG.
The tricky bit is rotate() which in SVG means something different if
there's an origin (you translate-then-untranslate it).
But this seems to work off-hand, and fix the reminder of bug 1906261.
Differential Revision: https://phabricator.services.mozilla.com/D215788
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=878346
gecko-commit: 246ed0e1a9345f7c55759d1a4e96cdfc89b8ba91
gecko-reviewers: longsonr, firefox-style-system-reviewers, zrhoffman