Improve automatic tessellation

[conal]

Parametric surfaces are automatically tessellated into an efficient infinite trie. Rendering selects one such tessellation to use, taking partial account of modeling transformation, by manipulating an error threshold. Currently, only scaling transformation modifies that threshold.

To do:

• Give the error threshold a clear semantics.
• Account for the combined model and view transforms, including translation.
• Account for the nature of the defined surface. Perhaps use derivative bounds.