Fix surface colormap discontinuity for circular colorscales

[etpinard]

fixes #940

This PR implements @dfcreative's gl-vis/gl-surface3d#8 patch in plotly.js. Big ups! 🍻

This PR adds logic for circular colorscale in surface traces as discussed in gl-vis/gl-surface3d#8 (comment)

I chose to not add a new colorscale attribute for the moment as the behavior for circular colorscale is on par with 2D heatmap and contour traces.

[rreusser]

Only potential blocker from my point of view: how many of the existing colormaps are circular? It seems preferable that the evaluation mode not suddenly change for most colormaps (IMO to a sometimes-desirable but usually-slightly-inferior method). It would be nice if it were sorta opt-in by choosing a colormap that's intentionally circular (or at least that it's not most of them that are affected).

[etpinard]

how many of the existing colormaps are circular?

Of the built-in plotly colorscales you mean? In that case: none.

It seems preferable that the evaluation mode not suddenly change for most colormaps

I disagree. To me, this PR is simply a bug fix.

Can you think of a use case where a user that sets a circular colorscale would want to display a gap in the resulting colormap?

[rreusser]

Ah okay. I'm 100% content then. If that's the case, then I agree that this is a perfectly good way to encode the intent, making this a nice little feature/bug fix. 👍🏼