st/nine: Move texture setting to nine_context_*

st/nine: Move texture setting to nine_context_*

And move samplers_shadow to nine_context.
Part of the refactor to move all gallium calls to
nine_state.c, and have all internal states required
for those calls in nine_context.

Signed-off-by: Axel Davy <axel.davy@ens.fr>