Original X11 device demuxer patch from Clemens Fruhwirth

- Build system integration is equivalent to RFC #6 patch
- Same rule applies to ffmpeg.c/allformats.[c|h]
- x11grab.c is from Clemens Fruhwirth except the x11 grab
  structure for libavformat registration is renamed to
  match build system integration.

Originally committed as revision 7293 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/allformats.c

@@ -166,6 +166,7 @@ void av_register_all(void)
     REGISTER_DEMUXER (WSAUD, wsaud);
     REGISTER_DEMUXER (WSVQA, wsvqa);
     REGISTER_DEMUXER (WV, wv);
+    REGISTER_DEMUXER (X11_GRAB_DEVICE, x11_grab_device);
     REGISTER_MUXDEMUX(YUV4MPEGPIPE, yuv4mpegpipe);
 
 #ifdef CONFIG_PROTOCOLS