allow model specific outline style

[dkoes]

Like with transparency, this will be per-style per-model since it is a material property. I've implemented the groundwork so we just need to set outline to true in the material, but have only implemented it with cartoons for now. Also, I haven't yet added support for custom outline styles (uses whatever the last set viewer style is).

Arguably, we should switch from this being a viewer style to a model style sort of thing.

[dllahr]

Excellent! Thank you.

[dkoes]

https://3dmol.org/tests/auto/generate_test.cgi?test=test28