Scroll trigger button mask from SDL2 (1: left, 2: middle, 4: right)

Set to 0 to deactivate the scrolling feature.

Property definitions

gamnit :: camera_control_linux $ EulerCamera :: camera_pan_mask=
	# Scroll trigger button mask from SDL2 (1: left, 2: middle, 4: right)
	#
	# Set to 0 to deactivate the scrolling feature.
	var camera_pan_mask = 2 is writable
lib/gamnit/camera_control_linux.nit:26,2--29,36