avcodec: disable threading by default

Threading does not currently work with some codec profiles and is thus
not suited as default. Threading is also known to deadlock in certain
circumstances, such as if no data is ever received (a rather common
scenario with network or DVB reception).

Fixed avcodec version should be checked before this is re-enabled.
Acked-by: Ilkka Ollakka <ileoo@videolan.org>
(cherry picked from commit 8d59ba01d33c96d61db5e0e7ab9b8a30b052ebab)

Conflicts:
	modules/codec/avcodec/avcodec.c

modules/codec/avcodec/avcodec.c

@@ -132,7 +132,7 @@ vlc_module_begin ()
     add_bool( "ffmpeg-hw", false, HW_TEXT, HW_LONGTEXT, false )
 #endif
 #if defined(FF_THREAD_FRAME)
-    add_integer( "ffmpeg-threads", 0, THREADS_TEXT, THREADS_LONGTEXT, true );
+    add_integer( "ffmpeg-threads", 1, THREADS_TEXT, THREADS_LONGTEXT, true );
 #endif