png: encoder: encode visible lines, but use pitch for offset

(cherry picked from commit fd1e287a11d678a8e131d59979e84fb4486c0c57)

modules/codec/png.c

@@ -407,7 +407,7 @@ static block_t *EncodeBlock(encoder_t *p_enc, picture_t *p_pic)
 
     for( int i = 0; i < p_pic->p->i_visible_lines; i++ )
     {
-        png_write_row( p_png, p_pic->p->p_pixels + (i * p_pic->p->i_visible_pitch) );
+        png_write_row( p_png, p_pic->p->p_pixels + (i * p_pic->p->i_pitch) );
         if( p_sys->b_error ) goto error;
     }