#381 use image DPI settings

[Michael Rice]

For maps and stamps.

most image file formats (I checked a random GIF banner
off a webpage) have a DPI setting.

If we could use that setting, we may be able to make a
better guess at how to use the map or stamp.

As suggested by Phergus on our boards, map DPI could be
used to set the grid spacing.

Stamp DPI could be used to determine the cells a stamp
would cover (to start).