swrast: Replace ImageOffsets with an ImageSlices pointer.

swrast: Replace ImageOffsets with an ImageSlices pointer.

This is a step toward allowing drivers to use their normal mapping paths,
instead of requiring that all slice mappings come from an aligned offset
from the first slice's map.

This incidentally fixes missing slice handling in FXT1 swrast.

v2: Use slice height helper function.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>