GDK W32: Apply HiDPI scale properly to monitors

GDK W32: Apply HiDPI scale properly to monitors

Previously HiDPI scale was retrieved and applied too late in the initialization
process to affect monitor size and monitor workarea size, but the code that
initializes these sizes *did* try to use the scale, even though it was always
getting scale=1.

To fix this, move the too-late code into monitor enumeration routine.
This also fixes a probable semantic bug where width and height were divided
by scale, again.

Now monitor and workarea should be in application pixels (i.e. divided by scale),
as intended.

https://bugzilla.gnome.org/show_bug.cgi?id=778835