usdShade: Use TfStaticData for shared UsdStageCache

Besides being recommended practice, this avoids a hang on
Windows that could occur when the static UsdStageCache member
was destroyed during process teardown. The UsdStage objects
stored in the cache would try to use TBB in their d'tors
after the TBB library had been unloaded and its worker
threads destroyed, leading to odd deadlocks. This most
recently showed up in testUsdChecker* but also affected
imaging tests, which were disabled primarily because of
this issue.

(Internal change: 2353331)

pxr/usd/usdShade/shaderDefParser.cpp

@@ -11,6 +11,8 @@
 
 #include "pxr/usd/usd/stageCache.h"
 
+#include "pxr/base/tf/staticData.h"
+
 PXR_NAMESPACE_OPEN_SCOPE
 
 TF_DEFINE_PRIVATE_TOKENS(
@@ -22,7 +24,7 @@ TF_DEFINE_PRIVATE_TOKENS(
     (usd)
 );
 
-UsdStageCache UsdShadeShaderDefParserPlugin::_cache;
+static TfStaticData<UsdStageCache> _StageCache;
 
 static
 NdrTokenMap
@@ -52,11 +54,10 @@ UsdShadeShaderDefParserPlugin::Parse(
     const std::string &rootLayerPath = discoveryResult.resolvedUri;
 
     SdfLayerRefPtr rootLayer = SdfLayer::FindOrOpen(rootLayerPath);
-    UsdStageRefPtr stage = 
-        UsdShadeShaderDefParserPlugin::_cache.FindOneMatching(rootLayer);
+    UsdStageRefPtr stage = _StageCache->FindOneMatching(rootLayer);
     if (!stage) {
         stage = UsdStage::Open(rootLayer);
-        UsdShadeShaderDefParserPlugin::_cache.Insert(stage);
+        _StageCache->Insert(stage);
     }
 
     if (!stage) {

pxr/usd/usdShade/shaderDefParser.h

@@ -15,8 +15,6 @@
 
 PXR_NAMESPACE_OPEN_SCOPE
 
-class UsdStageCache;
-
 /// \class UsdShadeShaderDefParserPlugin
 /// 
 /// Parses shader definitions represented using USD scene description using the 
@@ -40,9 +38,6 @@ class UsdShadeShaderDefParserPlugin : public NdrParserPlugin
 
     USDSHADE_API
     const TfToken &GetSourceType() const override;    
-
-private:
-    static UsdStageCache _cache;
 };
 
 PXR_NAMESPACE_CLOSE_SCOPE