Round map.colstate to nearest 5 when entering options menus

Also set map.tdrawback to true when leaving the menu.

This is to fix the interpolated color of the tower background
persisting, as well as making sure the menu background doesn't persist
when exiting.

desktop_version/src/Game.cpp

@@ -7762,4 +7762,5 @@ void Game::returntopausemenu()
     returntomenu(kludge_ingametemp);
     gamestate = MAPMODE;
     map.kludge_to_bg();
+    map.tdrawback = true;
 }

desktop_version/src/Input.cpp

@@ -2143,8 +2143,9 @@ void mapmenuactionpress()
         map.bg_to_kludge();
         game.kludge_ingametemp = game.currentmenuname;
 
-        map.nexttowercolour();
         map.scrolldir = 0;
+        map.colstate = ((int) (map.colstate / 5)) * 5;
+        map.nexttowercolour();
 
         // Fix delta rendering glitch
         graphics.updatetowerbackground();