Stacked plots should share a single GL context

[akhenry]

Is your feature request related to a problem? Please describe.
Browsers limit the total number of WebGL contexts available to us (by default in Chrome it's 16). We should look at ways that we can do more with this limited pool of Web GL contexts, and one option is to investigate sharing a single context between plots in a stacked plot.

Describe the solution you'd like
A stacked plot could conceivably share a single GL context, since all of the plots in a stacked plot are aligned vertically.