PhiloGL should not be globally bound to a single canvas

[D1plo1d]

It seems that PhiloGL is limited to a single instance rendering context in the moment due to it's use of the global gl object. This was not my intuition and should probably not be the case

I think we might be able to refactor the gl object to be per-instance and solve this issue but I'm still learning the PhiloGL code base. Can someone confirm if this is the case?

Use Case: my application. I need to render 2 WebGL contexts simultaneously.

See https://github.com/senchalabs/philogl/blob/master/src/core.js#L56

[D1plo1d]

Fixed. See Pull Request: #37