Negative grid coordinates are incorrect

[dougthor42]

Negative grid values are incorrect.

for positive values, everything is fine. However, global coord of -0.37 is being incorrectly calculated to be grid value 0, and global coord of -1.4 is being incorrectly calculated as grid value -1. The correct grid values for these are -1 and -2, respectively.

Should look at wm_utils.coord_to_grid() to fix.