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/scenelayers.h

@@ -267,9 +267,9 @@ class node_layers
 	{
 		return m_shapes;
 	}
-	// drops the definitions of the shapes of specified materials from the writable layer files on save.
+	// drops the definitions of the shapes of specified materials from the writable layer files on save; only from specified layers, if given.
 	// returns: amount of the shapes to drop, and of the ones which stay as their files can't be rewritten
-	std::pair<std::size_t, std::size_t> erase_shapes(std::set<material_handle> const &Materials);
+	std::pair<std::size_t, std::size_t> erase_shapes(std::set<material_handle> const &Materials, std::set<layer_handle> const *Layers = nullptr);
 	// indicates the scenario initialization (FirstInit) is being performed. Span: location of the directive.
 	// Infile: the directive comes straight from the layer file being loaded
 	void initialization(source_span const &Span, bool Infile);