swr/rast: Implement Early Rasterization optimization

swr/rast: Implement Early Rasterization optimization

Early Rasterization is an optimization for small triangles.

Scientific workloads often contain very small triangles that has non-zero
area and cannot be trivially rejected as falling between pixel centers,
but does not cover any pixel center. Those triangles can be initially
rasterized as early as in binner and rejected if they cover no pixels The
optimization can be disabled in compilation using KNOB_ENABLE_EARLY_RAST
option in knobs.h

The Early Rast is disabled by default.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>