Fix renderer.pick() miss return value

We were expecting null but Scratch returns false, TW returns -1 (what Scratch's comments say it should)
Let's handle both to ensure maximum compatibility.

Author: GarboMuffin

src/scaffolding/scaffolding.js

@@ -134,7 +134,7 @@ class Scaffolding extends EventTarget {
   _startDragging (x, y) {
     if (this._draggingId) return;
     const drawableId = this.renderer.pick(x, y);
-    if (drawableId === null) return;
+    if (drawableId === -1 || drawableId === false) return;
     const targetId = this.vm.getTargetIdForDrawableId(drawableId);
     if (targetId === null) return;
     const target = this.vm.runtime.getTargetById(targetId);