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

editor/editorGroundMesh.hpp

@@ -120,13 +120,15 @@ class ground_mesh
 	}
 
   private:
+	// NOTE: clamped before the conversion: a rectangle asked about can reach far beyond the mesh (to infinity even),
+	// and a float out of the range of int doesn't turn into anything sensible
 	int column(float const X) const
 	{
-		return std::clamp(static_cast<int>(std::floor((X - m_low.x) / m_cellsize)), 0, m_columns - 1);
+		return static_cast<int>(std::clamp(std::floor((X - m_low.x) / m_cellsize), 0.f, static_cast<float>(m_columns - 1)));
 	}
 	int row(float const Z) const
 	{
-		return std::clamp(static_cast<int>(std::floor((Z - m_low.y) / m_cellsize)), 0, m_rows - 1);
+		return static_cast<int>(std::clamp(std::floor((Z - m_low.y) / m_cellsize), 0.f, static_cast<float>(m_rows - 1)));
 	}
 	// cells the bounds of a triangle reach: first column, first row, last column, last row
 	std::array<int, 4> cells(triangle const &Triangle) const