demux: ogg: fix first oggds timestamps/drop regression

oggds uses 0 on first frame :/

(cherry picked from commit a09fbee518)

modules/demux/ogg.c

@@ -1012,6 +1012,10 @@ static void Ogg_UpdatePCR( demux_t *p_demux, logical_stream_t *p_stream,
     {
         /* We're in headers, and we haven't parsed 1st data packet yet */
 //        p_stream->i_pcr = VLC_TS_UNKNOWN;
+        if( p_stream->b_oggds && ogg_page_packets( p_oggpacket ) )
+        {
+            p_stream->i_pcr = VLC_TS_0 + p_ogg->i_nzpcr_offset;
+        }
     }
     else if( p_oggpacket->granulepos > 0 )
     {
@@ -1168,7 +1172,12 @@ static void Ogg_SendOrQueueBlocks( demux_t *p_demux, logical_stream_t *p_stream,
                 temp = temp->p_next;
                 tosend->p_next = NULL;
 
-                if( tosend->i_pts < VLC_TS_0 )
+                if( tosend->i_dts < VLC_TS_0 )
+                {
+                    tosend->i_dts = tosend->i_pts;
+                }
+
+                if( tosend->i_dts < VLC_TS_0 )
                 {
                     /* Don't send metadata from chained streams */
                     block_Release( tosend );

modules/demux/oggseek.c

@@ -678,12 +678,11 @@ int64_t Oggseek_GranuleToAbsTimestamp( logical_stream_t *p_stream,
                                        int64_t i_granule, bool b_presentation )
 {
     int64_t i_timestamp = -1;
-    if ( i_granule < 1 )
+    if ( i_granule < 1 - !!p_stream->b_oggds )
         return -1;
 
     if ( p_stream->b_oggds )
     {
-        if ( b_presentation ) i_granule--;
         i_timestamp = i_granule * CLOCK_FREQ / p_stream->f_rate;
     }
     else  switch( p_stream->fmt.i_codec )