unionRoundMask can cause first layer problems

Under some circumstances unionRoundMask requires re-flattening of the bottom of the model before slicing.

Consider:

`use
$fn = 36;
unionRoundMask( r = 1 , detail = $preview ? 3 : 10 , q= $preview ? 30 : 70, showMask = true ) {
cylinder (h=5, r=15);
cylinder (h=10, r=10);

}

translate([40,0,0])
union(){
cylinder (h=5, r=15);
cylinder (h=10, r=10);
}`

Examine the bottoms of both objects. Not a big problem, but does require attention if the fillet in near the base of the object.

Start with the unionRoundMask.scad entry point and reproduce the issue using the OpenSCAD example in the report, comparing its bottom surface with the plain union. Trace how the mask handles the first layer and re-flattening; done means the generated bottom no longer requires manual re-flattening when the fillet is near the base.

Written by the indexing model from the issue text.