Use legacy DefaultPrim logic if using legacy material scopes

Since the default prim logic was needed to determine the parent scope of
materials in the current material scope behavior, it isn't needed if
using the legacy material scope strategy

Author: dj-mcg

lib/mayaUsd/fileio/jobs/writeJob.cpp

@@ -222,12 +222,16 @@ bool UsdMaya_WriteJob::_BeginWriting(const std::string& fileName, bool append)
     MayaUsd::ProgressBarScope progressBar(8);
 
     // If no default prim for the exported root layer was given, select one from
-    // the available root nodes of the Maya scene. We take into account the excluded
-    // node types based on the export job arguments.
-    if (mJobCtx.mArgs.defaultPrim.empty()) {
-        MStringArray roots = GetExportDefaultPrimCandidates(mJobCtx.mArgs);
-        if (roots.length() > 0) {
-            mJobCtx.mArgs.defaultPrim = roots[0].asChar();
+    // the available root nodes of the Maya scene in order for materials to be
+    // parented correctly. We take into account the excluded node types based on
+    // the export job arguments. This is not required if using the legacy
+    // material scope.
+    if (!mJobCtx.mArgs.legacyMaterialScope) {
+        if (mJobCtx.mArgs.defaultPrim.empty()) {
+            MStringArray roots = GetExportDefaultPrimCandidates(mJobCtx.mArgs);
+            if (roots.length() > 0) {
+                mJobCtx.mArgs.defaultPrim = roots[0].asChar();
+            }
         }
     }