Remove reference to removed options -m -p
Be a little more permissive of geospec
Fix error resulting from corners intersection

Discovered via the command line:

    ffcast -vv -ssi

If you select two non-overlapping regions, region_intersect_corners()
doesn't actually check for this fact. We check this after, in the
set_region_vars_by_corners() function.
Use nameref instead of eval

To avoid "circular name reference", we hereby establish a convention to
always prefix a nameref variables with ref_:

    local -n ref_name=$1

The reason for using a prefix instread of a suffix is it's easier to
access those names via ${!prefix@} for debug purposes.
Cleanups and details