Remove game.gametimer in favor of game.frames

PR #279 added game.gametimer solely for the editor ghosts feature. It
seems that whoever originally wrote it (Leo for the now-dead VVVVVV:
Community Edition, I believe) forgot that the game already had its own
timer, that they could use.

The game timer does increment on unfocus pause (whereas this doesn't),
but that's a separate issue, and it ought to not do that.

desktop_version/src/RenderFixed.cpp

@@ -130,7 +130,7 @@ void gamerenderfixed(void)
     {
         if (map.custommode && !map.custommodeforreal)
         {
-            if (game.gametimer % 3 == 0)
+            if (game.frames % 3 == 0)
             {
                 int i = obj.getplayer();
                 GhostInfo ghost;