This makes it more similar to llvmpipe. It also allows us to let draw emit
code handle things like getting zeros for non-existing vs outputs
automatically. There probably isn't really any overhead either way, there isn't
really any "simply copy everything" code in the emit path it would copy each
attrib individually just the same. Likewise, we still do another mapping step
in softpipe as the layout may still not match exactly (same as in llvmpipe,
should probably nuke the pointless mapping in both drivers).

This fixes the piglit arb_fragment_layer_viewport no_gs/no_write tests.

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