i965: Fix BRW_VARYING_SLOT_PAD handling in the scalar VS backend.

i965: Fix BRW_VARYING_SLOT_PAD handling in the scalar VS backend.

We can't just break for padding slots.  Instead, treat them like
unwritten output variables, so we handle flushing and incrementing
urb_offset correctly.

Paul introduced the concept of padding slots back in 2011, but we've
never actually used them for anything.  So it's unsurprising that the
scalar VS backend didn't handle them quite right.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>