editor: tile paint and collision strokes are one undo step, no collision option for tiles (visual-only model copy in the build), freed gl buffers and stale pick ids, export level as gltf/glb with textures and vertex colours

docs/manual/19-shortcuts.md

@@ -157,6 +157,7 @@ Every menu command with its shortcut and a short description.
 | Import texture… |  | Import PNG/JPG images as textures (colour reduction, blend class, packing) |
 | Export level WAD… |  | Build the level and save the game file (.wad) somewhere |
 | Export selection as OBJ… |  | Mesh, UVs, vertex colours, textures (PNG) and collision for Blender |
+| Export level as glTF… |  | The whole level with textures and vertex colours as .glb / .gltf (Blender, viewers) |
 | Close editor | **Ctrl+Q** | Close the editor |
 
 ### Edit menu