packet: Add memory management *Packet functions

* Function '(*Packet).CopyProperties' is a wrapper for av_packet_copy_props
  It copes field values from one Packet to another.
  See: https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#gade00f67930f4e2a3401b67b701d5b3a2

* Function '(*Packet).MakeDateReferenceCounted' is a wrapper for av_packet_make_refcounted
  It makes sure the packet data is reference counted (in contrast to duplicated)
  See: https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#ga8a6deff6c1809029037ffd760db3e0d4

* Function '(*Packet).MakeDataWritable' is a wrapper for av_packet_make_writable
  It makes sure the packet data is writable (in contrast to reference counted)
  See: https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#gaaa304ffdab83984ac995d134e4298d4b

buffer.go

@@ -0,0 +1,8 @@
+package astiav
+
+// #include <libavutil/buffer.h>
+import "C"
+
+func isBufferWritable(buf *C.AVBufferRef) bool {
+	return C.av_buffer_is_writable(buf) != 0
+}

packet.go

@@ -1,6 +1,7 @@
 package astiav
 
 //#include <libavcodec/avcodec.h>
+//#include <libavcodec/packet.h>
 import "C"
 import (
 	"errors"
@@ -120,6 +121,21 @@ func (p *Packet) Clone() *Packet {
 	return newPacketFromC(C.av_packet_clone(p.c))
 }
 
+// https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#gade00f67930f4e2a3401b67b701d5b3a2
+func (p *Packet) CopyProperties(src *Packet) error {
+	return newError(C.av_packet_copy_props(p.c, src.c))
+}
+
+// https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#ga8a6deff6c1809029037ffd760db3e0d4
+func (p *Packet) MakeDataReferenceCounted() error {
+	return newError(C.av_packet_make_refcounted(p.c))
+}
+
+// https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#gaaa304ffdab83984ac995d134e4298d4b
+func (p *Packet) MakeDataWritable() error {
+	return newError(C.av_packet_make_writable(p.c))
+}
+
 // https://ffmpeg.org/doxygen/7.0/group__lavc__packet.html#gadfa708660b85a56749c753124de2da7d
 func (p *Packet) AllocPayload(s int) error {
 	return newError(C.av_new_packet(p.c, C.int(s)))

packet_test.go

@@ -85,4 +85,32 @@ func TestPacket(t *testing.T) {
 	b = []byte{}
 	require.NoError(t, pkt6.FromData(b))
 	require.Equal(t, b, pkt6.Data())
+
+	t.Run("CopyProperties", func(t *testing.T) {
+		pktCopy := AllocPacket()
+		require.NotNil(t, pktCopy)
+		defer pktCopy.Free()
+
+		require.NotEqual(t, pkt2.c.dts, pktCopy.c.dts)
+		err := pktCopy.CopyProperties(pkt2)
+		require.NoError(t, err)
+		// according to the documentation, all fields but "buf", "data", "size" should
+		// be copied, so let's check for example dts:
+		require.Equal(t, pkt2.c.dts, pktCopy.c.dts)
+	})
+
+	t.Run("MakeDataWritable", func(t *testing.T) {
+		pktClone := pkt2.Clone()
+		defer pktClone.Free()
+		err = pktClone.MakeDataWritable()
+		require.NoError(t, err)
+		require.True(t, isBufferWritable(pktClone.c.buf))
+	})
+
+	t.Run("MakeDataReferenceCounted", func(t *testing.T) {
+		pktClone := pkt2.Clone()
+		defer pktClone.Free()
+		err = pktClone.MakeDataReferenceCounted()
+		require.NoError(t, err)
+	})
 }