avcodec/vorbisdec: export skip_samples instead of dropping frames

pts may not be set on input packets, which could result in the entire stream
being discarded.
This reverts commit 8fc2dedfe6, reintroducing the
behavior it replaced but now allowing the caller to manually drop the preroll
samples by looking at the skip_samples side data at the start while ignoring it
on seek, by setting the skip_manual avctx flag.

Fixes ticket #10251.

Suggested-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit cbcc817353)

libavcodec/vorbisdec.c

@@ -38,6 +38,7 @@
 #include "codec_internal.h"
 #include "decode.h"
 #include "get_bits.h"
+#include "internal.h"
 #include "vorbis.h"
 #include "vorbisdsp.h"
 #include "vorbis_data.h"
@@ -134,7 +135,6 @@ typedef struct vorbis_context_s {
     av_tx_fn      mdct_fn[2];
 
     uint8_t       first_frame;
-    int64_t       initial_pts;
     uint32_t      version;
     uint8_t       audio_channels;
     uint32_t      audio_samplerate;
@@ -1839,13 +1839,7 @@ static int vorbis_decode_frame(AVCodecContext *avctx, AVFrame *frame,
 
     if (!vc->first_frame) {
         vc->first_frame = 1;
-        vc->initial_pts = frame->pts;
-    }
-
-    if (frame->pts == vc->initial_pts) {
-        *got_frame_ptr = 0;
-        av_frame_unref(frame);
-        return buf_size;
+        avctx->internal->skip_samples = len;
     }
 
     ff_dlog(NULL, "parsed %d bytes %d bits, returned %d samples (*ch*bits) \n",
@@ -1877,6 +1871,7 @@ static av_cold void vorbis_decode_flush(AVCodecContext *avctx)
                              sizeof(*vc->saved));
     }
     vc->previous_window = -1;
+    vc->first_frame = 0;
 }
 
 const FFCodec ff_vorbis_decoder = {