Providing option to restrict element-picking to a particular type

This came up when I was preparing the coding assignments for CMU's DDG course.

Currently, the element picker picks up on elements of all sorts, including vertices, edges, faces, halfedges. On the other hand, in some assignments, students only need to select vertices on a mesh. Vertex selection is still doable, but on larger meshes it can require zooming in to more precisely select vertices.

It seems we've discussed this problem before (just logging it here.) One thing I'll add is that it might be helpful if the user could restrict to a certain element type by holding down a key while clicking.

実装ファイル、テスト、エントリーポイントは指定されていません。まず element-picker のコードを見つけ、issue で参照されている以前の議論を確認してください。既存の picking の動作を失うことなく、提案されているキーを押し続ける操作も含め、ユーザーが選択した要素タイプに picking を制限できれば完了です。