Enable high-DPI (Retina) text rendering in Passage editors.

Disabling buffered drawing means text is rendered directly. This
could cause flickering, so if there' a way to increase the
resolution of the buffer rather than disabling it entirely,
that would probably be preferable.

Solution taken from:

https://groups.google.com/forum/#!topic/scintilla-interest/tj71w3UMj4s

This fixes the first item in issue #114