Emscripten webgl context

[juj]

Implements a HTML5 API for creating WebGL contexts. Adds support for creating multiple GL contexts on different canvases. So far tested that browser test suite passes.

[juj]

Implements #2559. Related to #2515, #2540, #643 and #644.

[juj]

One of the things I really hate with WebGL is that it only allows setting context creation parameters as true or false, i.e. depth: false/true, alpha: false/true, stencil: false/true, antialias: false/true. There's no current way to specify how many depth bits or multisamples one would like to have.

I was thinking whether to make the creation API possibly future-proof by not having these fields represent booleans but just integers in terms of number of bits to allocate, and currently treat anything nonzero as true.. That way in the future one could say the preferred bit depth with depth: 16 or depth: 24 or depth: 32. This would be preferable and useful optimization for e.g. Firefox OS mobile. However, it seems that the argument against that is "it's too difficult to implement", see http://www.khronos.org/webgl/public-mailing-list/archives/1407/msg00055.html

Another way would be to just have the depth/stencil/alpha as true/false, and in the future, if such feature is abailable, add new fields int depthBits, int stencilBits and so on.

[kripken]

these changes require bumping the version number

[kripken]

This looks good. Can you please check though that it does not affect perf? In particular I worry about storing properties on the context object like that (temp buffers, etc.). It shouldn't affect perf, but let's be sure.

[juj]

Rebased to latest.

Tested performance on both the Firefox packaged app we've been working on earlier this year, and on my 3D engine, and I cannot observe a difference in performance.