Win32: Fix a bug in fullscreen, and consolidate the fullscreen code into one function.

99% of the code is the same anyway.

The bug was created in https://github.com/hrydgard/ppsspp/commit/cf00fd1ac883cfe101607e73e68fd7a4ee30b394 by @vnctdj. Basically, if we start in fullscreen, we start with a borderless window with no menu, set to the size of the last windowed mode used (which is of course, not desirable. If I exit in fullscreen, I certainly expect it to begin in fullscreen again).
If nCmdShow is ignored now, should it be removed as a parameter?  This is the change meant to affect functionality, right?

-[Unknown]