iris: Remap stream output indexes back to VARYING_SLOT_*.

iris: Remap stream output indexes back to VARYING_SLOT_*.

Previously I had a hack in st/mesa to make it stop remapping
VARYING_SLOT_* into the naively compacted slots, which aren't
what we want.  But that wasn't very feasible, as we'd have to
update all drivers, or add capability bits, and it gets messy fast.

It turns out that I can map back to VARYING_SLOT_* in about 5 LOC,
so let's just do that.  It removes the need for hacks, and is easy.

This also fixes KHR-GL46.enhanced_layouts.xfb_capture_struct, which
apparently with my hack was still getting the wrong slot info.