MgCooker: Tile count completely wrong when override coordinates specified

[jng]

1. Select the Sheboygan tile set
2. Start a tiling session. Observe that it has about 127k tiles to process.
3. Stop the session and override its coordinates, cut down a few decimal places.
4. Re-run the tiling session. The session ends immediately, and most likely the calculated total is 0 or some very small number.

[jng]

In 9125:

Fix the row/col offset calculations when override extents are involved

Fixes #2745