codec/zvbi: Only reset subpicture in text mode

The subpicture reset, as stated in the comment, is only required when
rendering text-subtitles. In fact it causes an invalid image to be generated
and possibly output in non-text mode. Hence disable the picture recreation for
non-text mode.
Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 47012982d20cf537fc710edd14c9c9c4183d0a19)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/zvbi.c

@@ -372,7 +372,7 @@ static subpicture_t *Decode( decoder_t *p_dec, block_t **pp_block )
 
     if( !b_cached )
     {
-        if( p_sys->i_last_page != i_wanted_page )
+        if( p_sys->b_text && p_sys->i_last_page != i_wanted_page )
         {
             /* We need to reset the subtitle */
             p_spu = Subpicture( p_dec, &fmt, true,