st/nine: Use ff vertex shader when position_t is used

st/nine: Use ff vertex shader when position_t is used

When an application sets a vertex shader, we are supposed
to use it, and when no vertex shader are set, we are supposed
to revert to fixed function vertex shader.

It seems there is an exception: when the vertex declaration
has a position_t index, we should revert to fixed function
vertex shader.

Up to know we were checking if device->state.vs is set
to know whether to use programmable shader or not.

With this commit we determine whether we use programmable shader
or not when vertex shader/declaration are set, but
stateblocks do complicate things a bit.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>