allow contour only op chains with shadow disabled

src/kiri/mode/cam/op-shadow.js

@@ -41,9 +41,22 @@ class OpShadow extends CamOp {
         }
 
         if (skipTerrain) {
+            let { stock } = settings;
+            let { center } = stock;
+
             console.log("skipping terrain generation");
-            shadowBase = [ newPolygon() ];
+            shadowBase = [ newPolygon().centerRectangle(center, stock.x, stock.y) ];
             tzindex = [ ];
+
+            state.shadow = {
+                base: shadowBase,       // computed shadow union at base of part
+                holes: [],
+                skip: skipTerrain,
+                slices: [],             // legacy / transitional
+                stack: [],              // stack of shadow slices
+            };
+
+            return;
         }
 
         // distributed pre-fill shadow layer cache

src/kiri/mode/cam/slice.js

@@ -150,6 +150,10 @@ export async function cam_slice(settings, widget, onupdate, ondone) {
         onupdate(null,null,msg);
     }
 
+    function shadowAt(z) {
+        return widget.shadowAt(z);
+    }
+
     if (unsafe) {
         console.log("disabling overhang safeties");
     }
@@ -202,7 +206,7 @@ export async function cam_slice(settings, widget, onupdate, ondone) {
         setAxisIndex,
         setToolDiam,
         settings,
-        shadowAt(z) { return widget.shadowAt(z) },
+        shadowAt,
         slicer,
         stock,
         tabs,