Don't allow implicit conversions from QSSGShaderDefaultMaterialKey

... to QSSGDefaultMaterialPreparationResult.

Change-Id: I9c413cb0bb70ecbce46715ef9cf7194f21450edf
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/runtimerender/rendererimpl/qssgrendererimpllayerrenderpreparationdata_p.h

@@ -198,7 +198,7 @@ struct QSSGDefaultMaterialPreparationResult
     QSSGShaderDefaultMaterialKey materialKey;
     bool dirty;
 
-    QSSGDefaultMaterialPreparationResult(QSSGShaderDefaultMaterialKey inMaterialKey);
+    explicit QSSGDefaultMaterialPreparationResult(QSSGShaderDefaultMaterialKey inMaterialKey);
 };
 
 // Data used strictly in the render preparation step.