Pass `gl` context

[luruke]

Creating a gl context is expensive.

Currently unless a forceRendererString is passed, the lib creates a new context.
Maybe we can add an option to pass an existing context?

[TimvanScherpenzeel]

Sure that sounds like a good idea 👍

[TimvanScherpenzeel]

I've added the option glContext to pass in an existing WebGL context which avoids the internal creation of one.

See https://github.com/TimvanScherpenzeel/detect-gpu/blob/9d2fe31f514fedb6b18214117125ae8edb656a3f/index.html#L69-L89 for an example 👍

Thanks for your suggestion!

The option is available from 1.1.0 onwards on NPM