Glir queues at Canvas instead of Context.

Whole system is more canvas-centric. There is a notion
of an active canvas and canvasses make themselves active
and deactive at the right moments so that gloo objects
gets associated with the right canvas in an intuitive way.

Fixes #617 and fixes #600
cleaning. Less context. docs
Make glir parser per-context.
Merge pull request #620 from almarklein/canvas-bug

Glir queues at Canvas instead of Context.