Reset map.bypos when entering options menus in-game

Otherwise the menu background would have this rendering glitch where the
bypos of the in-game tower wouldn't divide easily and have a bunch of
jitters in an otherwise smooth but overall still somewhat smooth
background.

desktop_version/src/Input.cpp

@@ -2145,6 +2145,7 @@ void mapmenuactionpress()
 
         map.scrolldir = 0;
         map.colstate = ((int) (map.colstate / 5)) * 5;
+        map.bypos = 0;
         map.nexttowercolour();
 
         // Fix delta rendering glitch