build: disable encoding mode by default

Encoding mode uses deprecated API. See previous commit. Encoding mode
will stop working/compiling at some point in the future, so unless
someone fixes the encoding code, it will stay disabled by default.

(Note that the deprecations are not merged in FFmpeg yet, but they will
soon. They've been deprecated in Libav for a while now.)

wscript

@@ -325,6 +325,7 @@ iconv support use --disable-iconv.",
     } , {
         'name' : '--encoding',
         'desc' : 'Encoding',
+        'default': 'disable',
         'func': check_true,
     }, {
         'name': '--libbluray',