save more material information,

hold default material in geometry separated,
load diffuse color if texture cannot be opened,
fixed gamma correction on the texture,

QtMeshViewer/Header/GeometryEngine.h

@@ -28,6 +28,7 @@ private:
 	QVector<Material>* m_materials = Q_NULLPTR;
 	QVector<DrawInformation> m_drawList;
 	BoundingBox m_boundings;
+	Material* m_defaultMaterial;
 
 	void clearData();