Description of the desired feature

Lots of users are falling into the trap that they make a PDF map of a coarse image, or request a discrete color bar, and when viewing the result on macOS with Preview the image looks either blurry (coarse grdimage plots) or the CPT looks continuous (for discrete CPTs). These are decisions Apple make in Preview and we cannot fight that. However, here are some things we can do:

1. The grdimage man page could have a Note: paragraph where it explains that coarse images viewed in PDF may look blurry depending on your PDF viewer and OS - this is a known issue on macOS.
2. The colorbar man page could do the same. But, note that colorbar -Np would draw a discrete color bar using rectangles which would not be subject to the Apple defect. I suggest we make -Np for discrete colormaps the default. This adds just a tiny amount of more PS (draw rectangle versus a tiny image) but will avoid the most common complaint for this problems (discrete color bars).

Let me know what @GenericMappingTools/core think about this.