Gl-agg line visual

Converts work by @rougier and @rossant into a nicely-antialiased line visual.
Depends on #369
We should talk about the best API for these sorts of objects. As a developer (and actually as a user, too) I prefer a simpler API, where you just have one set of styles for all parts of the visual, and using multiple instances of visuals when you need different styles. This reduces complexity of cases we'd have to handle from an API end. (I also think this is how most visuals are currently coded.) Then hopefully we could get collections of visuals working appropriately to deal with drawing efficiency.