transcode: change default videobitrate to 0

change from default 800 to zero, as nowdays most of bitrate transcoding
is for streaming, and 800 ain't good default for that either. Also
thisways it's easier on check different encoding modes (cbr/qp/etc) on
video encoder modules.

I did quick look on different modules, but didn't spot anything obvious
that would break. I'll look ffmpeg and x264 more closely next.

modules/stream_out/transcode.c

@@ -171,7 +171,7 @@ vlc_module_begin ()
                 VENC_LONGTEXT, false )
     add_string( SOUT_CFG_PREFIX "vcodec", NULL, NULL, VCODEC_TEXT,
                 VCODEC_LONGTEXT, false )
-    add_integer( SOUT_CFG_PREFIX "vb", 800 * 1000, NULL, VB_TEXT,
+    add_integer( SOUT_CFG_PREFIX "vb", 0, NULL, VB_TEXT,
                  VB_LONGTEXT, false )
     add_float( SOUT_CFG_PREFIX "scale", 1, NULL, SCALE_TEXT,
                SCALE_LONGTEXT, false )