Hide frame and stop threads when the viewer window is closed (do not exit the application).
UI improvements:
* 'S' and 'Esc' hide settings dialog.
* 'F1' brings up a help page (as announced by text overlay when viewer opens).
* fancy text overlays giving feedback on mode and interpolation switch.
* Zoom using Meta + Scroll-wheel.
UI improvements:
* when toggling/selecting source, the corresponding box in the overlay flashes white as a visual cue.
* use max mipmap when switching sources or modes to be more responsive.