st/nine: Implement TEXCOORD special behaviours

texcoord for ps < 1_4 should clamp between 0 and 1 the values.

texcrd (texcoord ps 1_4) does not clamp and can be used with
two modifiers _dw and _dz that means the channels are divided
by w or z.
Implement those in shared code, since the same modifiers can be used
for texld ps 1_4.

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