Editor terrain: texture browser, undo, conversion from terrain files

- Terrain work mode shows a browser of the textures folder in place of the
  node bank; textures/materials can be dragged onto the palette entries
  (replacing the material), the material field and the water material field
- Ctrl+Z/Ctrl+Y for heightmap terrain changes: a brush stroke or an
  operation is one step of the editor history
- Ground conversion reads only the files of the terrain directives
  (.txtf with origin/rotate, or .btf without text), paints the chunks with
  the materials of the triangles, drops only the shapes of those files
- Fix: ground grids converted out-of-range floats to int, so whole-scene
  queries returned the triangles of a single cell (shifted chunks)
- Fix: terrain layers copied from textures already uploaded without their
  data stayed grey
- Fix: ImGui ID conflict between terrain tool radios and section headers
- Fix: layers of terrain .txtf files named with the scenery/ prefix and
  could not be saved
- Editor selftest commands for the terrain

scene/scenenode.h

@@ -99,6 +99,8 @@ public:
         gfx::geometry_handle geometry { 0, 0 }; // relative origin-centered chunk of geometry held by gfx renderer
         std::vector<world_vertex> vertices; // world space source data of the geometry
 		gfx::userdata_array userdata;
+        // terrain file the shape comes from, see scene::terrain_file::reference_of(); 0: none, or not known (only the editor asks)
+        std::uint16_t terrainfile { 0 };
     // methods:
         // sends content of the struct to provided stream
         void
@@ -152,6 +154,9 @@ public:
     // sets whether the shape is drawn with the translucent geometry (used by generated geometry)
     void
         translucent( bool const Translucent );
+    // notes the terrain file the shape comes from
+    void
+        source_file( std::uint16_t const Reference ) { m_data.terrainfile = Reference; }
     // data access
     shapenode_data const &
         data() const;