pcbnew editing filled zones snaps outside the grid

[staviq]

If you create a copper fill zone, and if a side has a size that is an odd number in grid units ( for example 3 grid dots wide ), and you click the edge, and then try to move it via the center round dot, the dot snaps to grid, even though it is off the grid, because the edge is an odd size, resulting in the whole edge being off the grid.

Steps to reproduce:

1. In pcbnew, create copper fill zone, 3x3 grid units.
2. Select edge, and drag it by the round center dot.
3. The edge is now off grid, and the sides are at an angle.