Zoom factor, default at 1.2, higher means more reactive zoom effect

Property definitions

gamnit :: camera_control_linux $ EulerCamera :: camera_zoom_mod
	# Zoom factor, default at 1.2, higher means more reactive zoom effect
	var camera_zoom_mod = 1.2 is writable
lib/gamnit/camera_control_linux.nit:23,2--24,38