This allows the user to set a thicker grid line without causing the
grid lines to become over dense, or just make the grid denser according
to preference.
The term "density" is modified to "minimum grid spacing", which is
the pixel spacing between adjacent grid lines.
The spacing settings now comes in along, with other GAL display settings,
as part of GAL_DISPLAY_OPTIONS observer functionality. To this end, the
setter in GAL interface is removed, as an external user should be
setting this parameter via the GAL_DISPLAY_OPTIONS interface.
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