If we're not the current fullscreen window and we don't want to be fullscreen, don't touch the current video mode

Fixes an assertion in qemu on macOS, which creates multiple hidden windows:
2023-03-07 09:56:55.304 qemu-system-i386[9931:2984587] *** Terminating app due to uncaught exception 'NSGenericException', reason: 'NSWindowStyleMaskFullScreen cleared on a window outside of a full screen transition. Called from (
	0   AppKit                              0x00000001968f77ac __25-[NSWindow setStyleMask:]_block_invoke + 140
	1   AppKit                              0x00000001968f76cc NSPerformVisuallyAtomicChange + 108
	2   AppKit                              0x00000001968f7580 -[NSWindow setStyleMask:] + 188
	3   libSDL2-2.0.0.dylib                 0x0000000106531328 Cocoa_SetWindowFullscreen + 568
	4   libSDL2-2.0.0.dylib                 0x0000000106501700 SDL_UpdateFullscreenMode + 1316
	5   libSDL2-2.0.0.dylib                 0x0000000106504270 SDL_SetWindowFullscreen_REAL + 328
	6   libSDL2-2.0.0.dylib                 0x000000010650117c SDL_FinishWindowCreation + 100
	7   libSDL2-2.0.0.dylib                 0x0000000106500998 SDL_CreateWindow_REAL + 1812
	8   libSDL2-2.0.0.dylib                 0x00000001063efd54 SDL_CreateWindow + 76
	9   qemu-system-i386                    0x00000001050aa600 sdl2_window_create + 192
	10  qemu-system-i386                    0x00000001050a9fa0 sdl2_2d_switch + 196
	11  qemu-system-i386                    0x0000000104e9c784 displaychangelistener_display_console + 524
	12  qemu-system-i386                    0x0000000104e9e36c register_displaychangelistener + 264
	13  qemu-system-i386                    0x00000001050ab6d0 sdl2_display_init + 304
	14  qemu-system-i386                    0x000000010505c870 qemu_init + 13380

src/video/SDL_video.c

@@ -1420,6 +1420,13 @@ static int SDL_UpdateFullscreenMode(SDL_Window *window, SDL_bool fullscreen)
         if ((window->last_fullscreen_flags & FULLSCREEN_MASK) == (window->flags & FULLSCREEN_MASK)) {
             return 0;
         }
+        if (!fullscreen) {
+            if (_this->SetWindowFullscreen) {
+                _this->SetWindowFullscreen(_this, window, display, SDL_FALSE);
+            }
+            window->last_fullscreen_flags = window->flags;
+            return 0;
+        }
     }
 
     /* See if there are any fullscreen windows */