racket/gui cocoa: preserve resolution of bitmaps as control labels

This change is mainly for Retina display mode.
racket/draw: add a backing-scale argument to bitmap constructors

Generalizes backing-scale support created for `make-platform-bitmap`
and Mac OS X in Retina mode so that any bitmap can be created with
a backing scale (except monochrome bitmaps or bitmaps with masks).
images/icons: add `backing-scale` arguments

By default, icons render with a backing scale of 2.