Special handling of global pixel grids plotted with grdimage -JQ

[PaulWessel]

Description of proposed changes

See issue #4335 for background.

This PR acts on the recommendation to simply watch out for this very special case in grdimage and impose a central meridian that is an even multiple of the grid spacing for global, pixel-registered grids plotted under -JQ. Specifically, this adjust the global region to align with the grid spacing.

I marked this WIP because there are two failures that I would like reviewers to have a look at:

1. globalgrid.sh: This is expected since the lower left plot is exactly the special case we are addressing and the region is simply adjusted accordingly. Thus the failure is simply two incompatible maps and we will just update the PS original.
2. SST.sh: This is flagged as failure for me but unclear why. The code changes are not exercised since -JG and look like tiny changes. We have had changes to CPTs but not this one. Not sure why but perhaps not important.