Initial support for shared drawing contexts

This should allow flambe to be used together with other frameworks (e.g. Starling) in the same drawing context.

- Allows the chosen platform to be initialised with a specific drawing context (canvas for HTML5, Context3D for Flash)

- Flambe should not perform destructive operations when the drawing context is flagged as shared.

- Exposes the renderer through `System` to allow more fine-grained drawing control.

Currently only the Flash/Stage3D platform uses the shared flag.