st/mesa: add double input support including lowering (v3.1)

st/mesa:  add double input support including lowering (v3.1)

This takes a different approach to previously, we cannot index into the
inputMapping with anything but the mesa attribute index, so we can't use
the just add one to index trick, we need more info to add one to it
after we've mapped the input.

(Fixed copy propgation and cleaned up a little)

v2: drop float64 format check, just attr->Doubles.
merge enable patch.
v3: cleanup code a bit.
v3.1: minor review fixups (comment, newline) (Ilia)

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dave Airlie <airlied@redhat.com>