add union notes

src/geo/polygons.js

@@ -342,6 +342,11 @@
      * Slice.doProjectedFills()
      * Print.init w/ brims
      *
+     * clipper is natively less efficient at merging many polygons. this iterative
+     * approach skips attempting to merge polys lacking overlapping bounding boxes
+     * and can quickly check if the attempt to union two polys outputs the same
+     * two input polys. the latter bit is the key to greater speed.
+     *
      * @param {Polygon[]} polys
      * @returns {Polygon[]}
      */

src/mode/fdm/slice.js

@@ -1034,7 +1034,7 @@
             return;
         }
 
-        let unioned = POLY.union(solids, undefined, true).flat(), // TODO verify
+        let unioned = POLY.union(solids, undefined, true).flat(),
             isSLA = (spacing === undefined && angle === undefined);
 
         if (solids.length === 0) return false;