Don't always declare frag shader INPUT[0] as fragment position.

Don't always declare frag shader INPUT[0] as fragment position.

We were doing this for the sake of softpipe and the tgsi intergrepter since
we always need the fragment position and W-coordinate information in order
to compute fragment interpolants.
But that's not appropriate for hardware drivers.
The tgsi interpreter now get x,y,w information from a separate tgsi_exec_vector
variable setup by softpipe.
The new pipe_shader_state->input_map[] defines how vert shader outputs map
to frag shader inputs.  It may go away though, since one can also examine
the semantic label on frag shader input[0] to figure things out.