Fix scenegraph labels not clearing text mesh

- If a model skin is applied to a model which replaces a valid text string with an empty string, the old string would continue to render because the text mesh was still valid.

src/scenegraph/Label3D.cpp

@@ -52,6 +52,8 @@ namespace SceneGraph {
 	{
 		//regenerate geometry
 		m_geometry->Clear();
+		m_textMesh.reset();
+
 		if (!text.empty()) {
 			m_font->GetGeometry(*m_geometry, text, vector2f(0.f));