Recenter the window when exiting fullscreen mode

This moves the window back to the center of the display
instead of to (0, 0) when exiting fullscreen mode, which
has the side effect of hiding the title bar if it is present.

system_sdl.c

@@ -377,6 +377,12 @@ SDL_Surface* SDL_COMPAT_SetVideoMode(int width, int height, int bitsperpixel, Ui
 
   FreeResources();
 
+  // When leaving fullscreen mode, X and Y coordinates should be recentered relative to the display.
+  if (flags ^ SDL_WINDOW_FULLSCREEN) {
+      g_lastx = SDL_WINDOWPOS_CENTERED;
+      g_lasty = SDL_WINDOWPOS_CENTERED;
+  }
+
   g_window = SDL_CreateWindow("oricutron", g_lastx, g_lasty,
                               g_width, g_height, flags);
   if (g_icon)