We already keep the context in an RC, might as well keep another copy :-)
This re-applies commit 9b0dfa14b5 but
replaces the Clone trait on Shader, which would lead to duplicate and
too early program deletion, with an Rc<Shader>.
Based on Olivier's suggestion, the text rendering primitive is created
by painting the text onto a temporary HTML canvas
element and binding that to a texture.
The GL bindings were missing one feature on the web side, the ability to
set texture image data from an HTML canvas element.
We're going to need that soon.
The bump to a newer version also came with some odd (but sensible)
source incompatibilities.
