lines3d fails if called more than once

[bwlewis]

repro:

library(threejs)
m = cbind(x=1:10,y=1:10,z=(1:10)/((1:10)^2))
x = scatterplot3js(m)                      # 10 points
y = lines3d(x, 1, 10, lwd=5, color="blue") # plus one blue line
z = lines3d(y, 2, 10, lwd=5, color="blue") # all lines gone! (bug)