Simplify Pipeline pass management

Simplify Pipeline pass management

Originally I planned to use Pipelines to bundle objects with effects like
environmentally mapped reflection.  However, that task was delegated to
effects themselves recently, so there's no longer need for passthrough
rendering in Pipeline.  The render_all function was also a nuisance, as
it required Pipelines to be treated differently from other Renderables.

Passes are now stored in a list instead of a map.  This allows multiple
passes with the same tag (think multipass lighting), and avoids the need
to store the pass order separately.