Add Make current to GraphicsContext

[ghost]

Simple solution for #30. Adds the make_current command to the GraphicsContext and exposes it to Server object so that the api user can select the thread that they wish to make the graphics context valid on.

[kvark]

Looks good to me. The only problem is that we loose safety when the user forgets to call make_current. If that doesn't block you, I'd prefer to wait for @bjz to review it as well.

[brendanzab]

Restarting the travis build.

[brendanzab]

I'm still not happy with how the context/graphics API stuff is structured - it will need a rethink.

The original thought behind keeping the server on the main thread was because I thought OpenGL wasn't threadsafe. But this could be too limiting. Encoding this into the type system and making it safe is hard :(

What use case is this blocking you on @csherratt?

[kvark]

I believe we should accept this, and start looking for ways to add safety at either run-time or compile time in regards to GL task movement. The fact that we don't know how to safely do it doesn't mean we should block it completely.

[brendanzab]

@kvark Agreed. Merged.