draw: implement proper primitive assembler as a pipeline stage

draw: implement proper primitive assembler as a pipeline stage

we used to have a face primitive assembler that we ran after if
the gs was missing but we had adjacency primitives in the pipeline,
lets convert it to a pipeline stage, which allows us to use it
to inject outputs (primitive id) into the vertices. it's also
a lot cleaner because the decomposition is already handled for us.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>