SVG Improvements

This version upgrades the SVG support. Specifically ArcTo works more reliably on iOS and Mac. This also fixes SVG parsing bugs that prevented loading many SVG files.

A few properties have been added to IImage to fetch its dimensions.

There is no Mac editor included in this release (yet) because of a Xamarin bug: https://bugzilla.xamarin.com/show_bug.cgi?id=32895