gallivm: support per-pixel min/mag filter in SoA path

gallivm: support per-pixel min/mag filter in SoA path

Since we can have per-pixel lod we should also honor the filter per-pixel
(in fact we didn't honor it per quad neither in the multiple quad case).
Do this by running the linear path and simply beating the weights into shape
(the sample with the higher weight is the one which should have been chosen
with nearest filtering hence adjust filter weight to 1.0/0.0 based on that).
If all pixels use nearest filter (either min and mag) then still run just a
nearest filter as this is way cheaper (probably around 4 times faster for 2d,
more for 3d case) and it should be relatively rare that pixels really need
different filtering. OTOH if all pixels would require linear don't do anything
special since the linear path with filter adjustments shouldn't really be all
that much more expensive than ordinary linear, and we think it's rare that
min/mag filters are configured differently so there doesn't seem much value
in trying to optimize this further.
This does not yet fix the AoS path (though currently AoS is only used for
single quads hence it could be considered less broken, just never honoring
per-pixel filter decision but doing it per quad).

v2: simplify code a bit (unify min linear and min nearest cases)

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