Fixes issue where UsdGeomPrimvar::IsIndexed() doesn't work

for index attrs with only authored timeSamples.  We now have
UsdResolveInfo for answering "has value" questions without actually
needing to read in an attribute value (in theory, though actually
pulling off that optimization will require some SdfAbstractData API
work, to allow USD to query a value's type, which in crate and other
formats can be discovered without actually reading in the full value).

Fixes #238

(Internal change: 1767403)

pxr/usd/lib/usdGeom/primvar.cpp

@@ -251,9 +251,11 @@ UsdGeomPrimvar::GetIndices(VtIntArray *indices,
 bool 
 UsdGeomPrimvar::IsIndexed() const
 {
-    // XXX update this to api that can directly see if an attribute is blocked.
-    VtIntArray dummy;
-    return _GetIndicesAttr(/*create*/ false).Get(&dummy);
+    UsdResolveInfo valueSource = _GetIndicesAttr(/*create*/ false).
+        GetResolveInfo(UsdTimeCode::EarliestTime());
+
+    return (valueSource.HasAuthoredValueOpinion() && 
+            !valueSource.ValueIsBlocked());
 }
 
 bool 

pxr/usd/lib/usdGeom/testenv/testUsdGeomPrimvar.py

@@ -183,6 +183,15 @@ def test_PrimvarAPI(self):
         self.assertTrue(len(u1.ComputeFlattened(2.0)) == 0)
         self.assertNotEqual(u1.ComputeFlattened(2.0), u1.Get(2.0))
 
+        # Ensure that primvars with indices only authored at timeSamples
+        # (i.e. no default) are recognized as such.  Manual name-munging
+        # necessitated by UsdGeomPrimvar's lack of API for accessing
+        # the indices attribute directly!
+        u1Indices = p.GetAttribute(u1.GetName() + ":indices")
+        self.assertTrue(u1Indices)
+        u1Indices.ClearDefault()
+        self.assertTrue(u1.IsIndexed())
+
         # Finally, ensure the values returned by GetDeclarationInfo
         # (on new Primvar objects, to test the GprimSchema API)
         # is identical to the individual queries, and matches what we set above