ps_1_3 uses Tx to pass in texture coordinates, but also as temporary
registers. ps_1_4 and ps_2_0 only use them for texture coordinates. This patch
gets rid of the Tx = fragment.texcoord[x] assign in all shader versions, and
doesn't even declare Tx in ps_1_4 and ps_2_0.

The <=ps_1_3 instructions know which kind of input they expect from the Tx
register, so the instruction handlers now know if they have to read the
tempreg Tx or the varying fragment.texcoord[x].