Fix debug grid (#30)

[Frugghi]

Should fix the debug grid around the 180th meridian.

[choefele]

Thanks! I hoped to get this fixed for a while.

It seems to work fine, but there's a problem with the outer cells:

• In settings, enabled debug, choose "80000+ annotations in the US" and set Margin Factor to -0.2
• Zoom in a couple of times with double click
• Pinch zoom out (sim: Alt-Click and drag mouse)

You can see that the outer cells are very long and seem to wrap around the other side of the earth. See attached screenshot.

Could you please have a look if you can fix this?

[Frugghi]

Now it works very well if margin factor is >= 0.0 but sometimes the grid doesn't show if margin factor < 0.0 (it's a MapKit bug?).