Viewport create

I've implemented viewport create and changed window_main to use viewport functions. In the process I've had to rejig one or two other files. rct2_exit is really useless right now but I still make a function call to it in a new error case. I think the main problem with rct2_exit is that it creates a WM_QUIT message that SDL does not listen to. At present its not really an issue.

This could cause a few new bugs to surface but I haven't noticed any from just a quick look through. One interesting thing i've noticed is that the park entrance viewport is constantly trying to change size every tic. I'm not sure what is causeing that and it doesn't have any negative side affects.

Next challenge is viewport_paint! I think that does all of the painting for the park so it may take a while.