Quartz improvements

This pull request bundles together a number of improvements to the Quartz (Mac OS X and iOS) renderer.

* More bitmaps formats for both Mac OS X and iOS.
* Text layout is more consistent with pango layout, and adapts better to fonts with strange dimensions e.g. Zapfino.
* Text rendering on modern iOS now uses CoreText, just like Mac OS X.
* Support for multicolored fonts such as emoji.
* General code quality improvements.