- DRY using .plot & .cleear with getViewport helper fn
- Dry .plot and .update with repeat helper fn
- compute viewport only once per subplot, as opposed to
  once per trace.

- so that gl-based trace can call their plot methods w/ an
  empty array of traces and just work.
- update and improve scatterlg visibility tests to reflect
  that `restyle(gd,visible,false)` no longer clear the context

- to not have to guard against visible!==true traces in _module.style
- to shortcut full list of modules in other places downstream

- Bar.setPositions mutates 'b' in bar trace calcdata,
  so to reinit 'b' during setPositions so that it doesn't
  conflit with Bar.calc

- a Axes.expend clone that does not append things to ax._min/ax._max,
  but instead returns two arrays, a min array and max array
  of potential data extremes

- pass gd, to look for _extremes in gd._fullData and eventually
  in layout container to can expand the axis ranges

- replacing Axe.expand
- for ErrorBars, we append the min/max arrays
  of the corresponding trace

- questionable commit, especially the part in autorange.js,
  but this doesn't make any test fail ?!?

- N.B. polar is the only subplot apart from cartesian that
  used Axes.expand and friends.

- by using ax.(_traceIndices, annIndices, shapeIndices),
  to keep track of traces, annotations and shapes plotted on
  axis ax. Adapt concatExtremes accordingly!

... by factoring out logic from findExtremes

Recycled commits from abandoned fast trace toggle PRs