This does two things:

1. Allows cross shader optimisations to be preformed on SSO
   programs that contain more than one stage (seems like I
   don't have any of these in my shader-db collection).

2. Allows us to write compilation errors to stderr. With this
   change I discovered a Deus Ex: MD shader was failing to
   compile (I've reported it to Feral).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>