Improve rendering of scattergl, splom and parcoords by implementing plotGlPixelRatio for those traces

[jonmmease]

This PR updates the scattergl and parcoords traces to honor the config.plotGlPixelRatio parameter. Increasing the plotGlPixelRatio value now increases the resolution of the WebGL rendered content of these plots, both live and when exported to static images.

Closes #5497, and is the first step of addressing plotly/Kaleido#58.

Also closes #5500.

Already working:

• 3D plots
• scatterploargl
• heatmapgl

Fixed:

• scattergl
• parcoords
• splom

TODO:

• Review and update baselines

cc #5395

Examples

Here are some before and after examples. All of these are exported to png images with a scale of 3 and plotGlPixelRatio of 6.

Open both images in a new tab to get the full effect.

Scattergl

Before

After

SPLOM

Before

(Note, there was some marker size distortion before)

After

Parcoords

Before

After

Note that parcoords lines are thinner because the linewidth must remain at width 1, due to webgl line limitations, instead of increasing in width with GlPixelRatio the way that scattergl can.

Other Change: parcoords text shadow color

(Moved to #5506)

cc @archmoj @nicolaskruchten @alexcjohnson on parcoords style change

[jonmmease]

Here are a two images comparing the interactive appearance of parcoords before and after

Before:

After:

Note that the lines look thinner in this PR because gl lines are fixed at width 1, and the rendering is now performed at twine the resolution (4x more pixels).
cc @nicolaskruchten

[PeterDeWeirdt]

This looks like an awesome improvement! Any idea for when we can expect these changes to be merged?

[nicolaskruchten]

I think we decided that this is a bugfix that we could do after 2.0 i.e. in 2.1.

also re parcoords: if there is a parcoords trace in the figure, ration back to 1 for all the regl-based traces.

[archmoj]

Resolved conflicts and added to v2.3.0 milestone.

[archmoj]

Thanks @jonmmease for the 💪 PR 🎖️
💃 💃

[archmoj]

Awaiting merge :)

[nicolaskruchten]

You can merge it @archmoj