svga: don't emit extraneous fs shadow code

svga: don't emit extraneous fs shadow code

Depending on the depth texture format, we may or may not have to
emit explicit fs code to do the shadow comparison.  Before, we
were emitting it more often than needed.

v2: check the actual texture format rather than the screen->depth.z16
field.  The screen->depth.z16, x8z24, s8z24 fields may not all be set
to a consistent set of depth formats.

Reviewed-by: José Fonseca <jfonseca@vmware.com>