Allow using Space Station tilecol -1

This tilecol conveniently lets players use one of the unpatterned Space
Station tilesets you see on the left side of tiles.png but never get to
use without Direct Mode.

It does have a few weird quirks, but it should be safe to use.

desktop_version/src/editor.cpp

@@ -1686,9 +1686,15 @@ void editorclass::clamp_tilecol(const int rx, const int ry, const bool wrap /*=
     const int tileset = room.tileset;
     int tilecol = room.tilecol;
 
-    int mincol = 0;
+    int mincol = -1;
     int maxcol = 5;
 
+    // Only Space Station allows tileset -1
+    if (tileset != 0)
+    {
+        mincol = 0;
+    }
+
     switch (tileset)
     {
     case 0: