avcodec: use qscale=3 as default if videobitrate isn't provided

If user doesn't give videobitrate use some reasonable default to get ok output
from avcodec transcode (for example mpeg4 video).

modules/codec/avcodec/avcodec.c

@@ -208,7 +208,7 @@ vlc_module_begin ()
                  ENC_QMAX_TEXT, ENC_QMAX_LONGTEXT, true )
     add_bool( ENC_CFG_PREFIX "trellis", false,
               ENC_TRELLIS_TEXT, ENC_TRELLIS_LONGTEXT, true )
-    add_float( ENC_CFG_PREFIX "qscale", 0,
+    add_float( ENC_CFG_PREFIX "qscale", 3,
                ENC_QSCALE_TEXT, ENC_QSCALE_LONGTEXT, true )
     add_integer( ENC_CFG_PREFIX "strict", 0,
                  ENC_STRICT_TEXT, ENC_STRICT_LONGTEXT, true )

modules/codec/avcodec/encoder.c

@@ -631,7 +631,7 @@ int OpenEncoder( vlc_object_t *p_this )
 
         p_context->mb_decision = p_sys->i_hq;
 
-        if( p_sys->i_quality )
+        if( p_sys->i_quality && !p_enc->fmt_out.i_bitrate )
         {
             p_context->flags |= CODEC_FLAG_QSCALE;
             p_context->global_quality = p_sys->i_quality;