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.

Currently only the Flash/Stage3D platform uses the shared flag.

Conflicts:
	src/flambe/System.hx
	src/flambe/platform/flash/FlashStage.hx
	src/flambe/platform/flash/Stage3DRenderer.hx
Improve support for embedding multiple instances of Flambe

- HtmlExternal now binds functions to the embed target, rather than `window`.  This is more
  consistent with the way it works for Flash and also allows interaction with multiple instances of
  HTML5 flambe.

- `flambe.embed` can take a callback that is fired when embedding is complete.  This could be used,
  for example, to embed multiple flambe instances in sequence.