gallivm: handle cube map arrays for texture sampling

gallivm: handle cube map arrays for texture sampling

Pretty easy, just make sure that all paths testing for PIPE_TEXTURE_CUBE
also recognize PIPE_TEXTURE_CUBE_ARRAY, and add the layer * 6 calculation
to the calculated face.
Also handle it for texture size query, looks like OpenGL wants the number
of cubes, not layers (so need division by 6).

No piglit regressions.

v2: fix up adding cube layer to face for seamless filtering (needs to happen
after calculating per-sample face). Undetected by piglit unfortunately.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com> (v1)