vd_lavc: more aggressive frame dropping for intra only codecs

Should speed up seeks.

(Unfortunately it's useless for backstepping. Backstepping is like
precise seeking, except we're unable to drop frames, as we can't know
the previous frame if we drop it.)

video/decode/lavc.h

@@ -37,6 +37,7 @@ typedef struct lavc_ctx {
     bool hwdec_failed;
     bool hwdec_notified;
 
+    bool intra_only;
     int framedrop_flags;
 
     // For HDR side-data caching

video/decode/vd_lavc.c

@@ -537,6 +537,9 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
     if (!lavc_codec)
         return;
 
+    const AVCodecDescriptor *desc = avcodec_descriptor_get(lavc_codec->id);
+    ctx->intra_only = desc && (desc->props & AV_CODEC_PROP_INTRA_ONLY);
+
     ctx->codec_timebase = mp_get_codec_timebase(vd->codec);
 
     // This decoder does not read pkt_timebase correctly yet.
@@ -1059,12 +1062,15 @@ static bool prepare_decoding(struct dec_video *vd)
         return false;
 
     int drop = ctx->framedrop_flags;
-    if (drop) {
-        // hr-seek framedrop vs. normal framedrop
-        avctx->skip_frame = drop == 2 ? AVDISCARD_NONREF : opts->framedrop;
+    if (drop == 1) {
+        avctx->skip_frame = opts->framedrop;    // normal framedrop
+    } else if (drop == 2) {
+        avctx->skip_frame = AVDISCARD_NONREF;   // hr-seek framedrop
+        // Can be much more aggressive for true intra codecs.
+        if (ctx->intra_only)
+            avctx->skip_frame = AVDISCARD_ALL;
     } else {
-        // normal playback
-        avctx->skip_frame = ctx->skip_frame;
+        avctx->skip_frame = ctx->skip_frame;    // normal playback
     }
 
     if (ctx->hwdec_request_reinit)
@@ -1095,6 +1101,9 @@ static bool do_send_packet(struct dec_video *vd, struct demux_packet *pkt)
     if (!prepare_decoding(vd))
         return false;
 
+    if (avctx->skip_frame == AVDISCARD_ALL)
+        return true;
+
     AVPacket avpkt;
     mp_set_av_packet(&avpkt, pkt, &ctx->codec_timebase);