fix case where ini directory is the presetdir directory

BinxelviewForm.cs

@@ -562,7 +562,8 @@ string saveIni(string path)
                     if (preset_dir.Length > 0)
                     {
                         string p = Path.GetFullPath(preset_dir);
-                        if (p.StartsWith(fulldir)) p = p.Substring(fulldir.Length+1);
+                        if (p == fulldir) p = "";
+                        else if (p.StartsWith(fulldir)) p = p.Substring(fulldir.Length+1);
                         sw.WriteLine("presetdir=" + p);
                     }
                     foreach (Preset pi in presets) // save current preset if it exists in the library

readme.txt

@@ -241,6 +241,7 @@ INI files provide the same options as the command line, with minor differences:
   - One option may be used per line, there is no leading - for an option like the command line.
   - Blank lines, or lines starting with # will be ignored.
   - Paths can be either absolute or relative to the INI file's directory.
+    Paths inside the INI directory (or subdirectories) will be automatically saved as relative.
   - The 'ini' option cannot be used to load an INI file from within an INI file.
 
 The default INI file location will be the first found in one of 3 locations, checked in order: