My last attempt to fix polygon offsets with the reversed viewport depth

My last attempt to fix polygon offsets with the reversed viewport depth
range used by the savage driver by negating ctx->MRD broke polygon
offsets with software fallbacks. This one adds a REVERSE_DEPTH parameter
to t_dd_tritmp.h (defaults to 0) that allows reversing polygon offsets
for hardware rendering but not for software fallbacks. For software
fallbacks depth values are reversed after polygon offsets have been
applied by the depth span functions.