Display source variables mapped to OpUndef as <undefined value>

* Undefined values are still 0xccccccccc, since that value can propagate through
  operations. But source variables mapped directly to an OpUndef are displayed
  in a more semantically clear way.

renderdoc/api/replay/shader_types.h

@@ -676,6 +676,13 @@ struct SourceVariableMapping
 )");
   VarType type = VarType::Unknown;
 
+  DOCUMENT(R"(A flag indicating if this mapping indicates an undefined value - the meaning of which
+is compiler defined but may mean either uninitialised data, unassigned values, or optimised away values.
+
+:type: bool
+)");
+  bool undefinedValue = false;
+
   DOCUMENT(R"(The number of rows in this variable - 1 for vectors, >1 for matrices.
 
 :type: int