mpegts: Make discard_pid() faster for single-program streams

When a stream contains a single program, there's no point in doing a
PID -> program lookup. Normally the one and only program isn't disabled,
so no packets should be discarded.

              Before          After
              Mean   StdDev   Mean   StdDev  Change
discard_pid()   73.8  9.4       20.2  1.5    +264.8%
Overall       2300.8 28.0     2253.1 20.6      +2.1%

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/mpegts.c

@@ -239,6 +239,17 @@ static int discard_pid(MpegTSContext *ts, unsigned int pid)
     int i, j, k;
     int used = 0, discarded = 0;
     struct Program *p;
+
+    /* If none of the programs have .discard=AVDISCARD_ALL then there's
+     * no way we have to discard this packet
+     */
+    for (k = 0; k < ts->stream->nb_programs; k++) {
+        if (ts->stream->programs[k]->discard == AVDISCARD_ALL)
+            break;
+    }
+    if (k == ts->stream->nb_programs)
+        return 0;
+
     for(i=0; i<ts->nb_prg; i++) {
         p = &ts->prg[i];
         for(j=0; j<p->nb_pids; j++) {