tgsi: add tgsi utility to transform a fragment shader to support aa point

tgsi: add tgsi utility to transform a fragment shader to support aa point

This adds a tgsi utility tgsi_add_aa_point to transform a fragment shader
to support anti-aliased wide point by computing the fragment distance from
the point center. This utility assumes the geometry shader is emitting
an extra generic output with point coord data. The semantic index of
this generic output is passed to the tgsi_add_aa_point utility.

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