tgsi: adds tgsi utility to transform a shader to support point sprite

tgsi: adds tgsi utility to transform a shader to support point sprite

This adds a tgsi utility tgsi_add_point_sprite to transform a geometry
shader to emulate wide points by drawing quads. This utility adds an
extra output for the original point position if the point position is
to be written to a stream output buffer. It also assumes the driver will
add a constant for inverse viewport scale after the user defined constants.

Reviewed-by: Brian Paul <brianp@vmware.com>