Don't show clipping caps for near/far clip planes.

[Tom Goddard]

Might want to not compute clipping caps for near/far clip planes. The most common case of using near/far clip planes is I think to look at atoms inside a molecule. If a density map or molecular surface is shown it is best not to cap it at the clip planes in that case. It is also very computationally expensive to continuously update the clip caps as the view is changed.

For clip planes fixed to the models default might remain to show caps.

Should be able to override the default (e.g. with the clip command: clip near -20 cap true).

Do ambient shadows update as the view moves with near/far clipping? That would be very expensive.

The motivation for this change is mostly performance, e.g. the problem discussed in #488.

[Tom Goddard]

Meshes have been made not to show clip caps by default.

Ambient shadows do recompute when clip planes moves otherwise interior would be darkened by clipped away parts of models which seems undesirable.

It is not clear if clip caps should be shown by default for opaque surfaces for near/far clip planes. Scene clip planes are not much different from near/far in their use.