Improved the assignation of UV slots for custom interpolators.

- Now reusing unused slots instead of simply adding them after the last used slot.
- Added missing calls to GatherExpressionsForCustomInterpolators()

#rb chris.bunner, jurre.debaare


#ROBOMERGE-OWNER: sebastien.lussier
#ROBOMERGE-AUTHOR: sebastien.lussier
#ROBOMERGE-SOURCE: CL 7321118 via CL 7321791
#ROBOMERGE-BOT: (v371-7306989)

[CL 7321793 by sebastien lussier in Main branch]

Engine/Source/Developer/MaterialUtilities/Private/MaterialUtilities.cpp

@@ -754,6 +754,15 @@ public:
 	{
 		return Material && Material->MaterialDomain == MD_Volume;
 	}
+
+	virtual void GatherExpressionsForCustomInterpolators(TArray<UMaterialExpression*>& OutExpressions) const override
+	{
+		if(Material)
+		{
+			Material->GetAllExpressionsForCustomInterpolators(OutExpressions);
+		}
+	}
+
 private:
 	/** The material interface for this proxy */
 	UMaterialInterface* MaterialInterface;