scattergl layering and errorbar issues

[alexcjohnson]

A couple of remaining scatter/scattergl differences, see https://codepen.io/alexcjohnson/pen/YemVOd

• fill tozero breaks in some weird ways - here it's filling to the wrong final y value, but if you slide trace1.y[0] up and down a little bit you see all sorts of odd behavior.
• fills are behind everything else, they should be on top of some of the lines/points/errorbars (note, I didn't include fill tonext in this example, but it makes fill layering even more complicated, see scattergl visible restyle fix #2442 (comment)) - Fixed in -> Scattergl improvements for: fill tozero with bad values fix, fills layering fix, add some line.shape values #3087
• x errorbars aren't supported? done -> scattergl errorbars fixes #2620

Initially I thought errorbar colors were also a scattergl issue, but scatter also doesn't get the colors I would have expected. That may have to be a v2 change at this point...

cc @dfcreative @etpinard

[tgoldina]

One more issue with error bars in scattergl, "markers" mode: error bars disappear on any relayout action: try switching to box select, zooming or panning in https://codepen.io/anon/pen/WJxKpP

[etpinard]

thanks for the report @tgoldina

[etpinard]

now in #4017