sd_lavc: work around bug in older ffmpeg releases

Older ffmpeg releases don't have ffmpeg git commit
50401f5fb7d778583b03a13bc4440f71063d319d, which fixes ffmpeg's
pkt_timebase check to reject its default "unset" timebase as invalid.
The consequence was that all non-PGS bitmap subtitle timestamps were
forced to 0.

Of course this hit _only_ shitty distros using outdated/badly maintained
ffmpeg releases, so this is not worth working around. I've already
wasted a lot of time on analyzing this dumb issue, and it could be
useful for bisecting, so don't drop pre-3.0 ffmpeg just yet.

Fixes #3109.

sub/sd_lavc.c

@@ -118,6 +118,7 @@ static int init(struct sd *sd)
     if (!ctx)
         goto error;
     mp_lavc_set_extradata(ctx, sd->codec->extradata, sd->codec->extradata_size);
+#if LIBAVCODEC_VERSION_MICRO >= 100
     if (cid == AV_CODEC_ID_HDMV_PGS_SUBTITLE) {
         // We don't always want to set this, because the ridiculously shitty
         // libavcodec API will mess with certain fields (end_display_time)
@@ -129,11 +130,14 @@ static int init(struct sd *sd)
         // which coincidentally won't be overridden by the ridiculously shitty
         // pkt_timebase code. also, Libav doesn't have the pkt_timebase field,
         // because Libav tends to avoid _adding_ ridiculously shitty APIs.
-#if LIBAVCODEC_VERSION_MICRO >= 100
         priv->pkt_timebase = (AVRational){1, AV_TIME_BASE};
         ctx->pkt_timebase = priv->pkt_timebase;
-#endif
+    } else {
+        // But old ffmpeg releases have a buggy pkt_timebase check, because the
+        // shit above wasn't bad enough!
+        ctx->pkt_timebase = (AVRational){0, 0};
     }
+#endif
     if (avcodec_open2(ctx, sub_codec, NULL) < 0)
         goto error;
     priv->avctx = ctx;