Display scaling?

[Po Lu]

Right now, the PGTK and NS ports apply a so-called "device scale", which
means that all drawing operations have an integral scale applied when
the user asks for it (for example, when he has a HiDPI monitor.)

The X and (presumably) W32 ports, however, only scale text on such
monitors, which means things that aren't text (such as the fringes and
boxes) are often displayed too tiny to be legible.

I think it would be nice to have such scaling on the other ports as
well.  We could probably apply the frame's display scale to the width of
its boxes when a face is being realized, and apply the scale to the the
fringe and its bitmaps as well.

It would be opt-in behaviour, of course.

WDYT?