get_screen_cons() was gated on DISPLAY being set, which only ever happens on
Linux, so headful runs on Windows and macOS generated fingerprints with no
monitor bound at all.
Fixes#425
screeninfo makes the process per-monitor DPI aware, so it reports physical
pixels, while Firefox lays windows out in CSS pixels. At 150% Windows scaling a
1920x1080 panel is 1280x720 CSS px, so bounding the fingerprint by the physical
size lets the window open 1.5x larger than the screen.
Refs #425