This is a quick fix until I get a random number generator into QF.
Mingw's RAND_MAX is only 0x7fff and so the (((rnd >> 10) & 63) - 31.5) / 63.0
used for the z component of origin and velocity would never go positive.
For now, change the 10 to 9 (reusing another bit from Y). I plan on
implementing a full 32-bit PRNG in QF so we always have a reliable
generator.