Blender Git Loki

Different integer overflow test for zbuffering, this due to mist+wire bug
that seems not to be solved on intel platforms.

Previously, a positive signed integer overflow addition was checked by
assuming it to become negative. Now it checks 'safe' with this version:

if(vergz > 0x7FFFFFF0 - dz) vergz= 0x7FFFFFF0;
else vergz+= dz;

Reason why using signed ints is cumbersome... overflows for unsigned are
much easer to find.