python: expose the attribute-less MeshData constructor.

So it's somewhat possible to really create meshes from scratch, and not
just annoyingly append to existing.

src/python/magnum/test/test_trade.py

@@ -823,6 +823,14 @@ class MeshData(unittest.TestCase):
         self.assertEqual(str(b), "MeshAttribute.BITANGENT")
         self.assertEqual(repr(b), "<MeshAttribute.BITANGENT: 3>")
 
+    def test_init_empty(self):
+        mesh = trade.MeshData(MeshPrimitive.TRIANGLES, 21)
+        self.assertIs(mesh.owner, None)
+        self.assertFalse(mesh.is_indexed)
+        self.assertEqual(mesh.primitive, MeshPrimitive.TRIANGLES)
+        self.assertEqual(mesh.vertex_count, 21)
+        self.assertEqual(mesh.attribute_count(), 0)
+
     def test(self):
         importer = trade.ImporterManager().load_and_instantiate('GltfImporter')
         # This adds extra attributes for joints and weights, don't want

src/python/magnum/trade.cpp

@@ -1072,6 +1072,9 @@ void trade(py::module_& m) {
         }, "Memory owner");
 
     py::class_<Trade::MeshData, Trade::PyDataHolder<Trade::MeshData>>{m, "MeshData", "Mesh data"}
+        .def(py::init([](MeshPrimitive primitive, UnsignedInt vertexCount) {
+            return Trade::MeshData{primitive, vertexCount};
+        }), "Construct an index-less attribute-less mesh data", py::arg("primitive"), py::arg("vertex_count"))
         .def_property_readonly("primitive", &Trade::MeshData::primitive, "Primitive")
         .def_property_readonly("index_data_flags", [](const Trade::MeshData& self) {
             return Trade::DataFlag(Containers::enumCastUnderlyingType(self.indexDataFlags()));