hls: Respect the different stream time bases when comparing dts

Also adjust the streams timestamps according to their start
timestamp when comparing. This helps getting correctly interleaved
packets if one stream lacks timestamps (such as a plain ADTS
stream when the other variants are full mpegts) when the others
have timestamps that don't start from zero.

This probably doesn't work properly if such a stream is
temporarily disabled (via the discard flags) and then reenabled,
and such streams are hard to correctly sync against the other
streams as well - but this works better than before at least.

The segment number restriction makes sure all variants advance
roughly at the same pace as well.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/hls.c

@@ -644,11 +644,28 @@ start:
         /* Check if this stream still is on an earlier segment number, or
          * has the packet with the lowest dts */
         if (var->pkt.data) {
-            if (minvariant < 0 ||
-                var->cur_seq_no <  c->variants[minvariant]->cur_seq_no ||
-               (var->cur_seq_no == c->variants[minvariant]->cur_seq_no &&
-                var->pkt.dts    <  c->variants[minvariant]->pkt.dts))
+            struct variant *minvar = c->variants[minvariant];
+            if (minvariant < 0 || var->cur_seq_no < minvar->cur_seq_no) {
                 minvariant = i;
+            } else if (var->cur_seq_no == minvar->cur_seq_no) {
+                int64_t dts     =    var->pkt.dts;
+                int64_t mindts  = minvar->pkt.dts;
+                AVStream *st    =    var->ctx->streams[var->pkt.stream_index];
+                AVStream *minst = minvar->ctx->streams[minvar->pkt.stream_index];
+
+                if (dts == AV_NOPTS_VALUE) {
+                    minvariant = i;
+                } else if (mindts != AV_NOPTS_VALUE) {
+                    if (st->start_time    != AV_NOPTS_VALUE)
+                        dts    -= st->start_time;
+                    if (minst->start_time != AV_NOPTS_VALUE)
+                        mindts -= minst->start_time;
+
+                    if (av_compare_ts(dts, st->time_base,
+                                      mindts, minst->time_base) < 0)
+                        minvariant = i;
+                }
+            }
         }
     }
     if (c->end_of_segment) {