Lack of thread safety around contexts

[cbaggers]

We havent really supported multiple contexts properly yet but we will do soon and so I needed a place to dump info on what I need to revisit.

• core/context/cepl-context.lisp#L6 The *contexts* variable
• core/context/cepl-context.lisp the object arrays (buffers/textures) shared between contexts

[cbaggers]

I've done a bit of a review of the globals. Not totally happy with this but it's an improvement in the obvious cases