Add shared context support to hosts

[cbaggers]

SDL2 is going to dictate the implementation as it only shares with a 'current' context. Most apis can take the context as an argument so who knows why they chose this approach. Not a real issue though.

I'm thinking that CEPL might have to be shared-context only when using multiple contexts. The reason is that pipelines hold the program id so we need to share to use the same id on all threads. Otherwise we need per context lookup tables from pipeline names to program-ids..ick.