swrast: fix broken _swrast_depth_clamp_span()

swrast: fix broken _swrast_depth_clamp_span()

The integer Z clamping range depends on the number of bits
in the Z buffer because that's the scale factor used when we
transform NDC coords by the viewport/depth range.

Fixes fd.o bug #25972 but only for Z buffers up to a depth
of 30 bits.  Beyond that we get into messy integer overflow
issues and things fall apart.