Cell Debug Overlay

Closes #5741 if merged.

Original message:
>This points out a possible flaw in our cell coordinates/offsets wrt TS (diamonds).
Buildings and units are on different cell grids	(mismatched by 0.5 cells).
Meaning buildings operate on a shifted grid (+/-)512 on x and y.
This is fine for mods that use rectangle (square) cells but turns things ugly for diamond TileShapes.
Main visual case is TS subcells vs structure foundation

In fact the error is all on the art side of things.
This makes fine-tuning sequence offsets by multiples of 6 much easier.

New dev commands:
```
/grid
/grid above
/grid below
/grid full
/grid half
/grid type
/grid range <integer>
/grid reset
```