Fixes https://developer.blender.org/T58659

The problem was that scaling of the grid was being applied twice. Once on line 775 (inside ED_view3d_grid_scale) and again on line 805.

So if one had a grid scale of 1.5, the snapping would occur in 1.5^2 = 2.25 increments not the expected 1.5 increments.