fix(editor): guard the places where the editor could crash outright

Found with an address and undefined behaviour sanitizer build, random
monkey sessions on the test sceneries and broken inputs:

- A deleted model left its pointer in the pick list of the renderer, the
  bound node of the panels, the bindings of the infrastructure and the
  selection: the next pick or panel frame read freed memory. The renderer
  forgets it, and one erase_model does the cleaning for all the places
  which deleted a model by hand.
- The plan band of a route read the vertex before the first one in the
  frame a drag was let go.
- A scenery which can't be opened goes back to the launcher with the
  reason in the log, a track of an unknown type or a tributary loads as
  a normal track or a river with a message, and a spline with no finite
  length or step gets a sane number of segments.
- Typed numbers which aren't (NaN, inf, ends 0 m or 1000 km apart, a
  vector of 1e30, a diverging track part of 5 km, a curve threshold of 0,
  a zero history size, radius norms below zero) are refused or clamped,
  where they made the geometry run away or divide by zero.
- Folders which can't be read and file names which aren't valid in the
  locale no longer throw out of the scans of sceneries, textures, sweeps,
  roads and signal plates; the scans skip them.
- The launcher: Ctrl+Del with no trainset selected, a dropped vehicle
  whose skin is gone and a text with a percent sign.
- A terrain mesh made for another resolution of its chunk isn't put into
  the chunk, and a slot is reused only for a mesh of the same size.
- Vehicles on a track changed by the editor are moved over a copy of the
  list they leave.
- The table of the keyboard shortcuts gives its stretched column a
  weight, else ImGui divides 0 by 0 in the first frame of the window.

launcher/textures_scanner.cpp

@@ -4,12 +4,13 @@
 
 void ui::vehicles_bank::scan_textures()
 {
-	for (auto &f : std::filesystem::recursive_directory_iterator("dynamic")) {
-		if (f.path().filename() != "textures.txt")
+	std::error_code error;
+	for (std::filesystem::recursive_directory_iterator f{"dynamic", std::filesystem::directory_options::skip_permission_denied, error}, end; f != end; f.increment(error)) {
+		if (f->path().filename() != "textures.txt")
 			continue;
 
-		std::fstream stream(f.path(), std::ios_base::binary | std::ios_base::in);
-		ctx_path = std::filesystem::relative(f.path().parent_path(), "dynamic/");
+		std::fstream stream(f->path(), std::ios_base::binary | std::ios_base::in);
+		ctx_path = std::filesystem::relative(f->path().parent_path(), "dynamic/", error);
 
 		std::string line;
 		while (std::getline(stream, line))