softpipe: fix mapping of "special" vs outputs

softpipe: fix mapping of "special" vs outputs

Unlike llvmpipe, softpipe always tells draw to emit the vertices as-is.
The two vertex layouts it calculates are a bit confusing, one which is just
used to tell draw to emit vertices as-is, and the other which has draw written
all over it but draw is completely unaware of and is used only to look up the
correct interpolation info later in setup.
Thus, the slots used are different to what llvmpipe does (I'm going to clean
up the confusing two layout stuff).

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>