st/nine: Rework texture data allocation

st/nine: Rework texture data allocation

Some applications assume the memory for multilevel
textures is allocated per continuous blocks.

This patch implements that behaviour.

v2: cache offsets

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Axel Davy <axel.davy@ens.fr>