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/07-meshes.md

@@ -7,7 +7,7 @@ collision and lit vertex colours, and its textures are packed onto texture pages
 
 The **library** works the other way round: it saves geometry from any level (original tiles,
 brushes and meshes, with their textures) so you can insert it into other levels. And **Export
-selection as OBJ** takes level geometry to Blender.
+selection as OBJ** takes level geometry to Blender, **Export level as glTF** the whole level.
 
 ## Supported files
 
@@ -178,6 +178,28 @@ it in Blender, then **File → Import mesh** it back (round trip).
 > **Note:** The exported textures are game data – fine for your own editing, please don't share
 > them.
 
+## Export the level as glTF
+
+**File → Export level as glTF…** writes the whole level – tiles, brushes and meshes – as one glTF
+2.0 file for Blender or any 3D viewer:
+
+* **.glb** (binary) or **.gltf** (text, everything in one file), chosen by the file type,
+* the level's texture pages as PNG pictures inside the file, the game's vertex colours
+  (light and shade) as vertex colours, transparent and double-sided parts as such,
+* unlit materials, so viewers show the baked colours like the game.
+
+| Option | Meaning |
+|---|---|
+| **Object models** | also the models of placed objects (coins, characters …) as the viewport shows them |
+| **Leave out hidden items** | items hidden in the editor (**H**) are not exported |
+| **Game orientation** | mirrors the level so that Blender and viewers show it the same way round as the game. Off: the same axes as the OBJ export and **Import mesh**, so the file can be imported again unchanged |
+
+Tiles with **Exclude from build** are left out. 1 unit is 1 cell.
+
+![glTF export dialog and the exported village read back by Import mesh](img/geo_gltf_export.jpg)
+
+> **Note:** The exported textures are game data – fine for your own use, please don't share them.
+
 ## Tips for good meshes
 
 * Model at the right scale: 1 cell is one grid square, the llama is about 0.6 cells tall. Use