Codechange: [SDL2] Split Start() in a few more functions

This makes it a bit easier to follow what is going on, and
allow future subdrivers to hook into a few of these functions.

Reworked the code slighly while at it, to return early where
possible.

src/video/sdl2_v.h

@@ -56,6 +56,7 @@ private:
 	void MainLoopCleanup();
 	bool CreateMainSurface(uint w, uint h, bool resize);
 	bool CreateMainWindow(uint w, uint h);
+	const char *Initialize();
 
 #ifdef __EMSCRIPTEN__
 	/* Convert a constant pointer back to a non-constant pointer to a member function. */