gallivm: do clamping of border color correctly for all formats

gallivm: do clamping of border color correctly for all formats

Turns out it is actually very complicated to figure out what a format really
is wrt range, as using channel information for determining unorm/snorm etc.
doesn't work for a bunch of cases - namely compressed, subsampled, other.
Also while here add clamping for uint/sint as well - d3d10 doesn't actually
need this (can only use ld with these formats hence no border) and we could
do this outside the shader for GL easily (due to the fixed texture/sampler
relation) do it here too just so I can forget about it.

v2: move border color clamping out of fetch texel. Also change it to clamp
the whole border vector at once (and use vectorized load of border color),
which saves a couple of instructions - needs some different handling of
mixed signed/unsigned formats so skip the per channel stuff and just derive
this from first channel except for special formats.

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