Flatten the top of baseplates

The current design of the baseplates with a sharp edge at the top is not
ideal for printability. Slicers try to approximate this sharp edge with
a thin perimeter line that is usually disjoint from the perimeters at
the grid intersections.  The end result is quite uneven.

Flatten this sharp edge by removing the top two layers of the baseplate,
leaving a flat surface with a width of roughly two perimeter lines on
top.  This is sliced much more consistently and the printed result also
looks a lot better.  The functional impact of not having a sharp edge on
top is neglible.

Author: Rahix

gridfinity-rebuilt-baseplate.scad

@@ -213,6 +213,10 @@ module gridfinityBaseplate(grid_size_bases, length, min_size_mm, sp, hole_option
             translate([0, 0, additional_height/2])
             cutter_screw_together(grid_size.x, grid_size.y, length);
         }
+
+        translate([0, 0, baseplate_height_mm])
+        // Cut off 2 layers from the top so the slicer can generate 2 perimeters side-by-side.
+        cube([size_mm[0] + 2, size_mm[1] + 2, LAYER_HEIGHT*4], center=true);
     }
 }