lavc: add xface image decoder and encoder

Based on libcompface code by James Ashton <James.Ashton@anu.edu.au>, and
relicensed to LGPL with the author's consent.

libavcodec/allcodecs.c

@@ -265,6 +265,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER (XAN_WC3, xan_wc3);
     REGISTER_DECODER (XAN_WC4, xan_wc4);
     REGISTER_ENCDEC  (XBM, xbm);
+    REGISTER_ENCDEC  (XFACE, xface);
     REGISTER_DECODER (XL, xl);
     REGISTER_ENCDEC  (XWD, xwd);
     REGISTER_ENCDEC  (Y41P, y41p);