swrast: always do span clipping in _swrast_write_rgba_span()

swrast: always do span clipping in _swrast_write_rgba_span()

It's possible for mis-behaving vertex programs to produce vertex data
with very large/NaN values.  This doesn't get handled reliably by the
clipper code so we may try to rasterize triangles that extend beyond
the viewport/window.  Always clip spans to avoid invalid memory accesses
later.