Fix negative texel wrapping in softgpu

Answer to the comment: no, it doesn't.  So it was crashing.  Adding abs() helped, but I found that using floor() was faster (we're only talking 2-3% speed -> 3.6% speed, though... I guess that's 20% faster, heh, but probably only for that scene.)

No longer crashes in Jeanne d'Arc, still looks awful.  Was wondering if maybe it showed the characters.

To confirm my math:
```
  -0.3 - floor(-0.3)
= -0.3 - -1.0
=  0.7

   0.6 - floor(0.6)
=  0.6 - 0.0
=  0.6
```

-[Unknown]