gallivm: do per-pixel lod calculations for explicit lod

gallivm: do per-pixel lod calculations for explicit lod

d3d10 requires per-pixel lod calculations for explicit lod, lod bias and
explicit derivatives, and we should probably do it for OpenGL too - at least
if they are used from vertex or geometry shaders (so doesn't apply to lod
bias) this doesn't just affect neighboring pixels.
Some code was already there to handle this so fix it up and enable it.
There will no doubt be a performance hit unfortunately, we could do better
if we'd knew we had a real vector shift instruction (with variable shift
count) but this requires AVX2 on x86 (or a AMD Bulldozer family cpu).
Don't do anything for lod bias and explicit derivatives yet, though
no special magic should be needed for them neither.
Likewise, the size query is still broken just the same.

v2: Use information if lod is a (broadcast) scalar or not. The idea would be
to base this on the actual value, for now just pretend it's a scalar in fs
and not a scalar otherwise (so, per-pixel lod is only used in gs/vs but same
code is generated for fs as before).

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