The merge commit to handle dynamic scenario frame size is broken

Ran into this while testing with my scenario:

Looks like the "ymaximum 500' command added as part of the last merge simply sets the size to 500 :| :| :|

I think it was simple defensive programming, so we can remove it to go back to a working version. I was gonna file an issue to discuss what the proper fix is, but I figure I might as well put up the one line "go back to what was happening before" fix. If it was addressing an actual issue, let me know and I'll see if I can find out why Renpy's idea of 'ymaximum' is simply 'y' :|