swr: set an explicit clear_rect if scissor is not enabled.

swr: set an explicit clear_rect if scissor is not enabled.

Fix regression of "no rendering" on simple apps like glxgears by
setting an explicit full surface clear_rect when scissor is not
enabled.

This regressed with commit 00173d91 "st/mesa: don't set 16
scissors and 16 viewports if they're unused" due to an assumption
that a default scissor rect is always set, which was the case prior
to this optimization.

Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>