I removed this mistakenly in 2dbc20e4

. I
actually thought it should not be necessary and a piglit run didn't show
any differences, but this shouldn't have been in there.
draw_prepare_shader_outputs() is in fact dependent on NEW_RASTERIZER.
The new polygon-mode-facing test indeed shows why this is necessary, there's
lots of invalid reads and writes with valgrind (also crashes without
valgrind), because the pre-pipeline vertex size doesn't match the
post-pipeline vertex size (note this won't help much with stages which don't
have the prepare hook which can grow the vertex size, in particular the wide
point stage, but this isn't used by llvmpipe). The test still won't pass, of
course, but it is only usage of uninitialized values now, which is much
less dangerous...
(Albeit I'm pretty sure for i915 it really is not needed anymore as it
doesn't care about the extra outputs and doesn't call
draw_prepare_shader_outputs().)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>