sceneconverter: reserve the mesh array before appending to it.

The number of items is known beforehand, so avoid unnecessary
reallocations. When mesh LODs are supported this won't be that clear
anymore, nevertheless it's never bad to pre-reserve.

src/Magnum/SceneTools/sceneconverter.cpp

@@ -858,6 +858,8 @@ well, the IDs reference attributes of the first mesh.)")
        args.value<Containers::StringView>("remove-duplicate-vertices-fuzzy") ||
        args.arrayValueCount("mesh-converter"))
     {
+        arrayReserve(meshes, importer->meshCount());
+
         for(UnsignedInt i = 0; i != importer->meshCount(); ++i) {
             Containers::Optional<Trade::MeshData> mesh;
             {