rtpenc_vp8: Use 15-bit PictureIDs

7-bit PictureIDs are not supported by WebRTC:
https://groups.google.com/g/discuss-webrtc/c/333-L02vuWA

In practice, 15-bit PictureIDs offer better compatibility.

Signed-off-by: Kevin Wang <kevin@muxable.com>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc_vp8.c

@@ -35,7 +35,8 @@ void ff_rtp_send_vp8(AVFormatContext *s1, const uint8_t *buf, int size)
     // partition id 0
     *s->buf_ptr++ = 0x90;
     *s->buf_ptr++ = 0x80; // Picture id present
-    *s->buf_ptr++ = s->frame_count++ & 0x7f;
+    *s->buf_ptr++ = ((s->frame_count & 0x7f00) >> 8) | 0x80;
+    *s->buf_ptr++ = s->frame_count++ & 0xff;
     // Calculate the number of remaining bytes
     header_size     = s->buf_ptr - s->buf;
     max_packet_size = s->max_payload_size - header_size;