fix niInstanceAggregationSceneIndex to handle paths that are

not namespace descendents when computing binding hash.
for a sibling path, walk ancestors to find the nearest native instance
root and lazily normalize it value to be relative to the instance root.

for model:modelPath in ModelAPI, we follow the deferred skinniing
pattern by relocating it to model:modelPath primvar in
UsdImagingNiPrototypePropagatingSceneIndex since it can be an arbitrary.
ancestor path. They are then aggregated automatically as instance
primvar on the instancer. this solves the hash difference as well as
allow it to survive instance aggregation, otherwise once aggregated,
only the value of the first instance will be available on the prototype.

add usdImaging unit test as well as a usdSkelImaging integration test.
add USDIMAGING_API export macro to methods used in test so that they
compile on windows.

(Internal change: 2420089)

Author: camelo918

pxr/usdImaging/usdImaging/CMakeLists.txt

@@ -261,6 +261,15 @@ pxr_build_test(testUsdImagingRerootingSceneIndex
     CPPFILES
         testenv/testUsdImagingRerootingSceneIndex.cpp
 )
+pxr_build_test(testUsdImagingNiAggregationNonDescendantPaths
+    LIBRARIES
+        usdImaging
+        hd
+        sdf
+        tf
+    CPPFILES
+        testenv/testUsdImagingNiAggregationNonDescendantPaths.cpp
+)
 pxr_build_test(testUsdImagingRootPrim
     LIBRARIES
         usdImaging
@@ -376,6 +385,12 @@ pxr_register_test(testUsdImagingRerootingSceneIndex
     COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testUsdImagingRerootingSceneIndex"
     EXPECTED_RETURN_CODE 0
 )
+pxr_register_test(testUsdImagingNiAggregationNonDescendantPaths
+    COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testUsdImagingNiAggregationNonDescendantPaths"
+    EXPECTED_RETURN_CODE 0
+    ENV
+        USD_ASSIGN_PROTOTYPES_DETERMINISTICALLY=1
+)
 pxr_register_test(testUsdImagingRootPrim
     COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testUsdImagingRootPrim"
     EXPECTED_RETURN_CODE 0

pxr/usdImaging/usdImaging/hdSchemaDefs.py

@@ -74,6 +74,9 @@
             ('assetName', T_STRING, {}),
             ('assetVersion', T_STRING, {}),
         ],
+        EXTRA_TOKENS = [
+            '(modelPathPrimvarName, "model:modelPath")',
+        ],
     ),
 
     #--------------------------------------------------------------------------

pxr/usdImaging/usdImaging/modelSchema.h

@@ -38,13 +38,16 @@ PXR_NAMESPACE_OPEN_SCOPE
     (assetIdentifier) \
     (assetName) \
     (assetVersion) \
+    ((modelPathPrimvarName, "model:modelPath")) \
 
 TF_DECLARE_PUBLIC_TOKENS(UsdImagingModelSchemaTokens, USDIMAGING_API,
     USD_IMAGING_MODEL_SCHEMA_TOKENS);
 
 //-----------------------------------------------------------------------------
 
 
+/// \class UsdImagingModelSchema
+///
 class UsdImagingModelSchema : public HdSchema
 {
 public: