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/06-geometry.md

@@ -267,6 +267,8 @@ used in the level** hides unused ones. Click a model to make it the current one,
 Painting onto a tile at the same height **swaps its model** instead of stacking a second tile.
 New tiles copy the vertex colours of an existing tile with the same model.
 
+One drag is one undo step: **Ctrl+Z** brings back all tiles a stroke painted over, erased or swapped.
+
 > **Tip:** Prefer swapping, moving or hiding original tiles over erasing them. Object scripts can
 > address tiles by their number, and erasing a tile shifts the numbers of all later tiles. The
 > status bar warns you when you erase an original tile.
@@ -326,7 +328,15 @@ change all tiles** (paint the model, every tile using it changes) or **Shared mo
 this tile** (the painted tile gets its own copy of the model first).
 
 Brushes have one material per face class (top, sides, bottom); tile models are painted per
-polygon. Tile collision cannot be switched off – paint it 30 or hide the tile instead.
+polygon. A painting drag is one undo step.
+
+### Tiles without collision
+
+To make a tile only visual, select it and tick **No collision** (Inspector, section **Build**; works
+for several selected tiles at once). The built level then uses a copy of the tile's model without
+collision polygons: the llama walks through the tile (a wall) or falls through it (a floor). Other
+tiles of the same model keep their collision. In the editor the tile no longer shows collision
+lines, and checks such as "no floor under the player start" ignore it.
 
 The layer **Invisible collision volumes** draws invisible walls, invisible blocks and colliders
 translucent red so you can see them while editing.