pgraph: Fix rare case of assertion in cull with ScissorEffect

Fixes #1344

panda/src/pgraph/cullTraverserData.I

@@ -114,6 +114,26 @@ get_node_path() const {
   return result;
 }
 
+/**
+ * Returns the transformed view frustum being used for culling at this level.
+ */
+INLINE GeometricBoundingVolume *CullTraverserData::
+get_view_frustum() const {
+  return _view_frustum.p();
+}
+
+/**
+ * Changes the transformed view frustum being used for culling at this level.
+ */
+INLINE void CullTraverserData::
+set_view_frustum(PT(GeometricBoundingVolume) view_frustum) {
+  if (_view_frustum == nullptr && view_frustum != nullptr && _cull_planes == nullptr) {
+    // In this case we will need up-to-date bounds but we may not have them.
+    _node_reader.check_cached(true);
+  }
+  _view_frustum = std::move(view_frustum);
+}
+
 /**
  * Returns the modelview transform: the relative transform from the camera to
  * the model.

panda/src/pgraph/cullTraverserData.h

@@ -65,6 +65,9 @@ public:
 
   INLINE NodePath get_node_path() const;
 
+  INLINE GeometricBoundingVolume *get_view_frustum() const;
+  INLINE void set_view_frustum(PT(GeometricBoundingVolume) view_frustum);
+
 PUBLISHED:
   INLINE CPT(TransformState) get_modelview_transform(const CullTraverser *trav) const;
   INLINE CPT(TransformState) get_internal_transform(const CullTraverser *trav) const;
@@ -79,6 +82,7 @@ PUBLISHED:
   void apply_transform(const TransformState *node_transform);
 
   MAKE_PROPERTY(node_path, get_node_path);
+  MAKE_PROPERTY(view_frustum, get_view_frustum);
 
 public:
   bool is_instance_in_view(const TransformState *instance_transform, const DrawMask &camera_mask) const;

panda/src/pgraph/scissorEffect.cxx

@@ -278,7 +278,7 @@ cull_callback(CullTraverser *trav, CullTraverserData &data,
   PT(GeometricBoundingVolume) frustum = make_frustum(lens, frame);
   if (frustum != nullptr) {
     frustum->xform(modelview_transform->get_inverse()->get_mat());
-    data._view_frustum = frustum;
+    data.set_view_frustum(frustum);
   }
 }