Textures: implement separate S,T clamping.

This allows you to clamp only the S or T co-ordinate by prefixing the
clamping type keyword with 's' or 't'.

You may specify sNoClamp and tEdgeClamp, for example; but you may not use
both zeroClamp with alphaZeroClamp in the same shader stage since both set
different border colours and only one border colour can be set.