sprites3d should allow different shapes for different points

[dmurdoch]

Currently sprites3d(..., shapes = x) allows x to be a vector of shape ids, but all shapes will be plotted at each point. It would be desirable to plot different shapes at each point, the way pch3d() can do with 2d shapes.

One way to do this would be to allow x to be a list of vectors, with each list entry treated as x is currently treated, but each point using a different list entry.