Add a new API to Pipeline that ties Renderables and passes together

Add a new API to Pipeline that ties Renderables and passes together

The old API is now deprecated and will eventually go away.  There are
two reasons for this change:

1. Pipeline was a sort of pseudo-scene, and in many simple use cases it
was not clear which should be used to hold Renderables.  Scenes are
intended as grouping primitives.

2. It was possible to add multiple passes with the same tag and different
rendering state, but there was no way to add a Renderable for only one of
them.  This led to tags doubling as a way to filter different passes at
the level of individual Renderables.  The intended purpose of tags is to
select between different presentations of a Renderable.