The option set pm3d border {line-properties} draws bounding lines around each
quadrangle as it is rendered. Normally this is used in conjunction with the
depthorder option to approximate hidden line removal.
Note that the global option set hidden3d has no effect on pm3d plots.
Default line properties (color, width) optionally follow the keyword border.
These defaults can be overridden later in an splot command.
Example of recommended usage:
set pm3d at s depthorder border lw 0.2 lt black
unset hidden3d
unset surf
splot x*x+y*y linecolor rgb "blue" # otherwise it would be black
