VideoRenderers: Set backbuffer size to full virtual screen size when not in D3DFS.

No more need to reinitialize the renderer when changing displays and will allow you to render the video across multiple monitors.
DX9AllocatorPresenter: Reset the device when the present mode changes.

Do it after the desktop display mode has been changed. That covers the changes in virtual desktop size too.  The other renderers already do this.
AppSettings: Turn off "Reinitialize when changing displays" by default on Vista+.

It's no longer as important now that the backbuffer is large enough to cover the entire Desktop. It's still needed for VSync though, so keep it on by default for XP.