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.
In order to support multiple font sizes, etc. we have to deal with the
fact that the glyphs for a string may not end up all in the same atlas
texture. Therefore splitting of glyph runs by texture is necessary.
This works but could be improved further by sharing the vertex arrays
and just re-binding to a different texture and drawing the run's
triangles by index.
