HLSL: Emulate write-to-output on return-from-entry-point, for return value.

This fixes issue #487 and #480.
It also correctly handles output parameters from the entry point.

hlsl/hlslParseHelper.h

@@ -196,6 +196,7 @@ protected:
     TString currentCaller;        // name of last function body entered (not valid when at global scope)
     TIdSetType inductiveLoopIds;
     TVector<TIntermTyped*> needsIndexLimitationChecking;
+    TVariable* entryPointOutput;
 
     //
     // Geometry shader input arrays: