Tweak odd cell count grids

[JShep1]

With the addition of snap to grid, we wish the snap to grid button to always align the object to the visible grid. In the case that the grid has an odd cell count, the grid realigns to center itself about the origin. This will currently result in a mismatch between where the object snaps to and the visual grid. After some discussion and considering alternatives (disallow odd grid cell count, making snap to grid align to the odd cell count grid), it seems the most desired/valuable solution to this scenario is that grids with odd cell counts should still intersect the origin as does a grid with an even cell count, but an additional row and column should be appended on an arbitrary side (likely +x, +y), rather than the whole grid shifting to make itself even about the origin on all sides.

[JShep1]

Issue resolved in ign-rendering#71