Rotation matrix read from head_tracker, reusing the same structure as a buffer

Property definitions

gamnit :: cardboard $ App :: _java_rotation_matrix
	# Rotation matrix read from `head_tracker`, reusing the same structure as a buffer
	private var java_rotation_matrix = new JavaFloatArray(16) is lazy
lib/gamnit/depth/cardboard.nit:51,2--52,66