Memory usage improvements

[publixsubfan]

Summary

• Defer buffering objects to the GPU until they're actually used in a pending frame
• Use indexed buffers to hold ordering curve
• Don't draw cones when cone_scale is 0 (PrepareOrderingCurves() uses Arrow3 to draw the no-arrow curve)

With these changes, I get around 0.97GB of memory usage for the command in #236, which puts us almost at where we were at version glvis-4.0.

Things to try:

• Remove second texture coordinate - this should drop us down from 32 to 16 bytes/vertex when storing VertexTex due to alignment
• Defer generation of element/vertex/edge numberings until they're toggled on - now 0.72GB

[tzanio]

@kanye-quest, the current failure is not due to that, but you probably want to merge master for its macos CI fixes

[tzanio]

• Should we mention it in CHANGELOG? -- done in 5b54bee

[tzanio]

@v-dobrev and @tomstitt -- can you please review when you get a chance?

[tomstitt]

Awesome work @kanye-quest !!