* video plugins that don't handle rescaling themselves have to let the

* video plugins that don't handle rescaling themselves have to let the
  video_output thread know about resizing events (with VOUT_SIZE_CHANGE event).
  (this part needs some clean-up but it is basically working for the X11 and
   SDL plugins).

* fixed fullscreen for the SDL plugin.

* xmga and sdl now switch to fullscreen on double-click.