Canvas 2D uses too many contexts

[Avaer Kazmer]

We use Skia accelerated rendering in Exokit. This uses GL contexts, one per canvas.

Unfortunately sites out there are using many canvas 2d's -- more than some systems allow in terms of GL context creation per window.

This should be fixable by having a single GL context that covers all of the 2d canvases, with only the textures shared.

The only thing we might need to be careful about is to make sure we reset the Skia context when switching textures. However, Skia might track this itself when switching textures.