gallivm: support array textures

gallivm: support array textures

Support 1d and 2d array textures (including shadow samplers),
and (as a side effect mostly) also shadow cube samplers.
Seems to pass the relevant piglit tests both for sampling and rendering
to (though some require version overrides).
Since we don't support render target indices rendering to array textures
is still restricted to a single layer at a time.
Also, the min/max layer in the sampler view (which is unnecessary for GL)
is ignored (always use all layers).

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