gallivm: implement fully accurate corner filtering for seamless cube maps

d3d10 requires that cube corners are filtered with accurate weights (that
is, the weight of the non-existing corner texel should be evenly distributed
to the other 3 texels). OpenGL does not require this (but recommends it).
This requires us to use different filtering code, since we need per-texel
weights which our 2d lerp doesn't (and can't) do. And of course the (now
per element) weights need to be adjusted too for it to work.
Invoke the new filtering code whenever there's an edge to keep things simpler,
as it will work for edges too not just corners but of course it's only needed
with corners.
More ugly code for not much gain but at least a hacked up cubemap demo
shows very nice corners now... Not sure yet if and how this should be
configurable...

v2: incorporate feedback from Jose, only use special corner filtering code
when there's a corner not when there's only an edge (as corner filtering code
is slower, though a perf difference was only measureable when always
forcing edge code). Plus some minor style fixes.

Reviewed-by: Jose Fonseca <[email protected]>

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