danger. major redo of core fdm slicer line connection algorithm. seems to handle non-manifold parts better. seems faster on torture tests. too good to be true?

src/geo/base.js

@@ -479,7 +479,7 @@
         // default # of decimal places in generated gcode
         gcode_decimals : 4,
         // heal disjoint polygons in slicing (experimental)
-        bridgeLineGapDistance : 0.0001,
+        bridgeLineGapDistance : 0.5,
         // Bounds default margin nearTo
         // Polygon.offset mindist2 offset precision
         precision_offset : 0.05,

src/geo/line.js

@@ -20,7 +20,7 @@
      * @constructor
      */
     function Line(p1, p2, key) {
-        if (!key) key = [p1.key, p2.key].join('-');
+        if (!key) key = [p1.key, p2.key].join(';');
         this.p1 = p1;
         this.p2 = p2;
         this.key = key;

src/geo/polygons.js

@@ -272,7 +272,7 @@
     }
 
     function flatten(polys, to, crush) {
-        if (!to) to = [];
+        to = to || [];
         polys.forEach(function(poly) {
             poly.flattenTo(to);
             if (crush) poly.inner = null;
@@ -399,7 +399,7 @@
              for (j=i+1; j<out.length; j++) {
                  if (!out[j]) continue;
                  union = out[i].union(out[j], minarea, all);
-                 if (union) {
+                 if (union && union.length) {
                      out[i] = null;
                      out[j] = null;
                      if (all) {