Work around SDL2 bug where VSync hint only applies on renderer creation

Ugh, this is terrible and stupid and I hate myself for it.

Anyway, since the SDL2 VSync hint only applies when the renderer is
created, we have to re-create the renderer whenever VSync is toggled.
However, this also means we need to re-create m_screenTexture as well,
AND call ResizeScreen() after that or else the letterbox/integer modes
won't be applied.

Unfortunately, this means that in main(), gameScreen.init() will create
a renderer only to be destroyed later by graphics.processVsync().
There's not much we can do about this. Fixing this would require putting
graphics.processVsync() before gameScreen.init(). However, in order to
know whether the user has VSync set, we would have to call
game.loadstats() first, but wait, we can't, because game.loadstats()
mutates gameScreen! Gahhhhhh!!!!

@leo60228 suggested to fix that problem (
https://github.com/TerryCavanagh/VVVVVV/pull/220#issuecomment-624217939
) by adding NULL checks to game.loadstats() and then calling it twice,
but then you're trading wastefully creating a renderer only to be
destroyed, for wastefully opening and parsing unlock.vvv twice instead
of once. In either case, you're doing something twice and wasting work.

desktop_version/src/main.cpp

@@ -217,6 +217,11 @@ int main(int argc, char *argv[])
 
     //Moved screensetting init here from main menu V2.1
     game.loadstats();
+
+    // FIXME: Thanks to having to work around an SDL2 bug, this destroys the
+    // renderer created by Screen::init(), which is a bit wasteful!
+    // This is annoying to fix because we'd have to call gameScreen.init() after
+    // game.loadstats(), but game.loadstats() assumes gameScreen.init() is already called!
     graphics.processVsync();
 
     if (game.skipfakeload)