Contribs: use FFmpeg by default

See #13777 (double-free in h264)
See #11510 (oma not working in libav)
See #13744 (PGS not working in libav)
See #13625 (MPJEG)
See #12945 (flv)
See H265 not working:
http://people.videolan.org/~jb/tmp/Luca/milkgirls1080p.mov-crf_26-preset_slow.mkv

contrib/src/ffmpeg/rules.mak

@@ -2,7 +2,7 @@
 
 #Uncomment the one you want
 #USE_LIBAV ?= 1
-#USE_FFMPEG ?= 1
+USE_FFMPEG ?= 1
 
 ifdef USE_FFMPEG
 HASH=fe47cba