Add an option to use the corner of the pixel to compute the footprint

This is a proposed solution for #915.
I followed @mdboom's suggestion and added a keyword to allow using the corner of the pixel instead of the center. In this way we keep the rest of the functionality in the function and it is backward compatible.
Any objections to doing it this way, @cdeil?

The old function is still present but deprecated. If this is accepted I'll add an entry to CHANGES.