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).

(cherry picked from commit 20f32dd168a37dde277edae8aee90906e79dcef4)
Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>

modules/codec/avcodec/avcodec.c

@@ -215,7 +215,7 @@ 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

@@ -627,7 +627,7 @@ 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;