Inconsistent axis anchoring behavior in scattergl

[cpsievert]

https://codepen.io/cpsievert/pen/xdjgXo

Change the trace type to "scatter" (in both traces) -- the yanchor of the xaxis changes

[etpinard]

I'm impressed that anything subplot related works in gl2d.

It's definitely not a good idea to use subplots in gl2d at the moment.

[etpinard]

related to #1200

[etpinard]

Will be fixed by @dfcreative 's regl-based scattergl work 🎉

[dy]

4c22e0c adds mock test

[etpinard]

done in #2258