avformat/mpegts: cache PID discard values

discard_pid can be quite expensive, so let's cache it and recalculate it on
every packet start.

ffmpeg -y -i samples/MPEG-VOB/sdtv/RAI.ts -c copy -map 0:v:0 -map 0:a:0 -f mpegts /dev/null

Before:
   1685 decicycles in handle_packet,  523483 runs,    805 skips

After:
    883 decicycles in handle_packet,  523505 runs,    783 skips
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/mpegts.c

@@ -91,6 +91,7 @@ struct MpegTSFilter {
     int es_id;
     int last_cc; /* last cc code (-1 if first packet) */
     int64_t last_pcr;
+    int discard;
     enum MpegTSFilterType type;
     union {
         MpegTSPESFilter pes_filter;
@@ -2474,8 +2475,6 @@ static int handle_packet(MpegTSContext *ts, const uint8_t *packet)
     int64_t pos;
 
     pid = AV_RB16(packet + 1) & 0x1fff;
-    if (pid && discard_pid(ts, pid))
-        return 0;
     is_start = packet[1] & 0x40;
     tss = ts->pids[pid];
     if (ts->auto_guess && !tss && is_start) {
@@ -2484,6 +2483,10 @@ static int handle_packet(MpegTSContext *ts, const uint8_t *packet)
     }
     if (!tss)
         return 0;
+    if (is_start)
+        tss->discard = discard_pid(ts, pid);
+    if (tss->discard)
+        return 0;
     ts->current_pid = pid;
 
     afc = (packet[3] >> 4) & 3;