FI-51 contour plot fill

[monfera]

Changes:

Related to contourgl fill:

• changes to gl-vis/gl-contour2d
  • triangle fix from @mikolalysenko
  • use fill color
  • include packages
  • changed example to more fully exercise the code
• changes to plotly.js
  • use gl-contour2d 1.1.2
  • changes to convert.js for the color fill setup
  • jasmine test cases (they only expect fill contours to work without raising an error)

Related to preparation for contourgl fill work:

• adding test utils for setup / teardown (removed for the time being, in favor of a TBD solution)
• refactoring of gl_plot_interact_basic_test using Plotly.purge (residual todo item from the already merged FI-30)

Planned or possible follow-up PRs once the gl-contour2d tessellation issue is solved (post the current v1.1.2):

• bump gl-contour2d version in package.json
• image based test cases for contourgl fill / line (now it would 'lock in' problematic artifacts)
• use of more standard surface test patterns (2D wave interference, saddle, noise etc.)
• improving similarity of appearance with the non-GL contour e.g. line thickness/color
• consider oversampling for a much finer grid, because gl-contour2d doesn't handle curves/splines, and the SVG code isn't directly reusable because the SVG implementation properly relies on splines achievable with the path mini-language of the d attribute
• depending on the solution to the tessellation issue as well as the possible oversampling above, consider refactoring opportunity for sharing more code between GL convert and the non-GL contour

[monfera]

[etpinard]

I'm ok with the boolean fill option here, as convertColorscale is only used in this file. But @monfera, I hope you agree that convertColorscale(fullTrace, { fill: true }); would have been cleaner and more robust.

[monfera]

@etpinard Glad to make this small change!

[etpinard]

No need for now. 👍

[etpinard]

Great. This is a solid patch for contourgl. Thank you very much for your efforts @monfera 🍻