MRView: ensure all width/height fetches use convenience functions

This will make it easier to track issues with Retina displays.
MRView: scale width/height by device pixel ratio
MRView & Retina displays: viewport now set using devicePixelRatio()

This should sort out most of the issues on MacOSX with Retina displays,
as discussed in issue #42. There will probably be some remaining issues
with font rendering - at the very least they will probably not be
rendered at Retina resolution.  Needs testing by someone with a Retina
display...
Merge pull request #73 from jdtournier/retina_display

Retina display