pass WebGL context to functions rather than relying on `var gl`

[Jonplussed]

I noticed the generated code relies upon a required global var gl to supply the WebGL context to the generated functions. What do you think about a change to the parser than instead adds an initial argument allowing gl to be passed as an argument?

While this would be cumbersome initially, this could allow the higher-level functions that rely on the generated code to operate in some sort of WebGL monad that could supply the context automatically, keeping a similar same interface while retaining functional purity.

If this is a direction you see your library going, I'd be happy to attempt a patch.

Thanks!