Refactor warp dir switching to separate function

This is so the same code can be used to go in reverse instead of
copy-pasting it.

desktop_version/src/editor.h

@@ -190,6 +190,7 @@ class editorclass{
   void switch_tilecol(const bool reversed);
   void clamp_tilecol(const int rx, const int ry, const bool wrap);
   void switch_enemy(const bool reversed);
+  void switch_warpdir(const bool reversed);
 
   bool load(std::string& _path);
   bool save(std::string& _path);