Fix some issues with perspective-corrected interpolation.

Fix some issues with perspective-corrected interpolation.

In mesa_to_tgsi.c, use TGSI_INTERPOLATE_PERSPECTIVE by default (to match
post-transform vertex info convention).  More to be done there...
In sp_quad_fs.c, interpolate W in addition to Z.  This fixes the divide
by zero happening in perspective_interpolation() tgsi_exec.c
As it was, we were only getting perspective correction of texture coords
used by the TGSI_TEX instruction since it does a homogeneous divide.
Other coords/varyings were incorrect.