Asynchronous SVG

[classilla]

https://bugzilla.mozilla.org/show_bug.cgi?id=759252

Not sure how this would perform on the uniprocessor systems, but it should be quite nice on the G5 and dual G4. I guess it wouldn't be any worse than now.

[classilla]

We'd need https://bugzilla.mozilla.org/show_bug.cgi?id=1190881 and deps for it to work, apparently.

[classilla]

Actually, for FPR1, land
https://bugzilla.mozilla.org/show_bug.cgi?id=1354080
and
https://bugzilla.mozilla.org/show_bug.cgi?id=1351986 (or the equivalent)
and then do the rest a little later.

[classilla]

We're not going to take the throbber after all, but we should take the asynchronous SVG patches.