bug#37689: Fringe pixmaps, widgets, etc. look ridiculously tiny in hidpi screen

[Carlos Pita]

> For example, if scaling factor > 1.5 make everything x2, if > 2.5 then
> x3, etc. It seems quite simple to achieve those integer scalings.

Or maybe, safer:

1. >= 3  then  3x
2. >= 2  then  2x
3. Otherwise 1x

This way you are sure that the icon will fit the fringe. And anyway
intermediate resolutions won't usually be 1.9x but 1.5x  instead
(although experimental fractional scaling configuration dialogs
currently offer 1.25x and 1.75x).