pull out all rgl dependencies into separate nat3d package

[jefferis]

This is what e.g. vegan/analogue have done with:

• http://cran.r-project.org/web/packages/vegan3d/index.html
• https://github.com/vegandevs/vegan3d
• http://cran.r-project.org/web/packages/analogueExtra/index.html

The reasoning is that there are problems with the rgl build e.g. on snow leopard that have not been resolved and that in general, it seems a waste to load rgl/X for functionality that does not depend on rgl e.g. when running on a cluster.

[jefferis]

A (possible) alternative is to set the rgl.useNULL option on package startup if we are in non-interactive mode.

[jefferis]

But that would still not solve the issue that rgl is frequently not available in some macosx configs.

[jefferis]

This solves the rgl load issue, but still leaves rgl as a dependency at build time, which can cause problems.