llvmpipe: support array textures

llvmpipe: support array textures

This adds array (1d,2d) texture support to llvmpipe.
Though probably should do something about 1d array textures requiring gobs
of memory (this issue is not strictly limited to arrays but it is probably
worse there).
Initial code by Jakob Bornecrantz <jakob@vmware.com>

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