draw: fix points with negative w coords for d3d style point clipping

draw: fix points with negative w coords for d3d style point clipping

Even with depth clipping disabled, vertices which have negative w coords
must be discarded. And since we don't have a proper guardband implementation
yet (relying on driver to handle all values except infs/nans in rasterization
for such points) we need to kill them off manually (as they can end up with
coordinates inside viewport otherwise).

v2: use 0.0f instead of 0 (spotted by Brian).

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