change the way rotation offsets are handled on the belt. appears to fix multi-part + support + anchor causing floating parts

src/mode/fdm/slice.js

@@ -332,8 +332,9 @@
                 // also find max width of first 5 layers
                 let start;
                 let minx = Infinity, maxx = -Infinity;
+                let peek = 0;
                 for (let slice of slices) {
-                    if (slice.index < 5) {
+                    if (slice.groups.length && peek++ < 5) {
                         for (let poly of slice.topPolys()) {
                             minx = Math.min(minx, poly.bounds.minx);
                             maxx = Math.max(maxx, poly.bounds.maxx);