This shares a global SharedContextWebgl among all instances of CanvasTranslator.
The goal is that regardless of how many windows are open, we only have to pay the
startup costs and shader compilation times for SharedContextWebgl once. In the
event that all CanvasTranslators are gone, the SharedContextWebgl is kept around
while its internal caches and textures are discarded to avoid significant memory
usage when no canvases are in use, while at the same time saving on startup
costs the next time a first live CanvasTranslator is created.
Differential Revision: https://phabricator.services.mozilla.com/D205977
