Tweaks the MaterialAsset loading logic to continue to see if the matDefinition already points to an existing object(to avoid needlessly re-executing files over and over), but also validate other cases, and ensures that if we DO have an existing definition, we still process and load it in the asset itself properly.

Engine/source/T3D/assets/MaterialAsset.h

@@ -73,6 +73,7 @@ public:
    enum MaterialAssetErrCode
    {
       ScriptLoaded = AssetErrCode::Extended,
+      DefinitionAlreadyExists,
       Extended
    };