KDE sometimes removes the Xft.dpi resource when it would be set to the
X11 default value of 96, causing GLFW to fall back to a value calculated
from the core display sizes in pixels and mm in a desktop environment
that supports Xft.dpi.

This moves to a hardcoded fallback value of 96 on the assumption that
there are more people running KDE with 96 DPI than there are people
running desktop environments that do not support Xft.dpi.

All of this is terrible please send help.

Fixes #1578.

(cherry picked from commit 75294462b354d147aa5914c30e3005551cc04e4a)