Startup positioning / size improvements

This was happening way too late in `OnMainFormShow()` before - there was always a noticable delay between FD showing  (in 800x600, since that was the default and it typically doesn't change when you're running it maximized) and it actually maximizing.

The startup now feels a lot more performant due to this change - might even have a real effect besides the perceived one, since the initial resizing (resizing is probably expensive?) isn't needed anymore.