[usdImaging] UsdImagingPiPrototypePropagatingSceneIndex now uses

a hash of the string representation of SdfPath, rather than of the
memory address of the internal SdfPath node, when generating names
for instancers, in order to provide stable results run-to-run.

(Internal change: 2386278)

Author: blevin

pxr/usdImaging/usdImaging/piPrototypePropagatingSceneIndex.cpp

@@ -404,8 +404,13 @@ _InstancerObserver::_InstancerHash(const SdfPath &instancer) const
     // re-rooted path of this prototype contains the instancer hash, so we
     // actually compute a chain of hashes if we have nested point instancers.
     //
-
-    const size_t h = TfHash::Combine(instancer, _propagatedPrototype);
+    // We use a hash of the path string representation, rather than the
+    // SdfPath instance, in order to provide a stable result run-to-run.
+    // Note that SdfPath's hash method uses the memory address of the
+    // heap allocated path node, which will vary.
+    const size_t h = TfHash::Combine(
+        instancer.GetString(),
+        _propagatedPrototype.GetString());
 
     return TfToken(TfStringPrintf("ForInstancer%zx", h));
 }