ext/sdl/sdlvideosink.*: Fix output stride copying, so that it displays correctly on framebuffer devices that don't ma...

Original commit message from CVS:
* ext/sdl/sdlvideosink.c: (gst_sdlvideosink_supported),
(gst_sdlvideosink_xoverlay_set_xwindow_id),
(gst_sdlvideosink_deinitsdl), (gst_sdlv_process_events),
(gst_sdlvideosink_event_thread), (gst_sdlvideosink_initsdl),
(gst_sdlvideosink_destroy), (gst_sdlvideosink_create),
(gst_sdlvideosink_setcaps), (gst_sdlvideosink_show_frame),
(gst_sdlvideosink_change_state),
(gst_sdlvideosink_navigation_send_event):
* ext/sdl/sdlvideosink.h:
Fix output stride copying, so that it displays correctly on
framebuffer devices that don't match our implict GStreamer stride
arrangement.

Fix locking things. Offer XOverlay only when SDL is running against
X. Make non-scaled (and ugly) embedding work via X Overlay. It can't
actually match the embedded window size because there's no way to
figure out what size that should be from the XOverlay interface.
See comment in sdlvideosink.c