add xdecode for reuse Frame object

add xdecode function for decoding to reuse Frame Object, prevent python GC using large of memory.

demo:

import av

in_container = av.open(file="test.flv", format='flv')

for packet in in_container.demux():
	if packet.stream.type == "video":
		for frame in packet.stream.codec_context.xdecode(packet):
			print(frame, frame.format)
	elif packet.stream.type == "audio":
		for frame in packet.stream.codec_context.xdecode(packet):
			print(frame, frame.format)

av/audio/frame.pyx

@@ -89,7 +89,8 @@ cdef class AudioFrame(Frame):
     cdef _init_user_attributes(self):
         self.layout = get_audio_layout(0, self.ptr.channel_layout)
         self.format = get_audio_format(<lib.AVSampleFormat>self.ptr.format)
-        self._init_planes(AudioPlane)
+        if self.planes is None:
+            self._init_planes(AudioPlane)
 
     def __repr__(self):
         return '<av.%s %d, pts=%s, %d samples at %dHz, %s, %s at 0x%x>' % (

av/codec/context.pyx

@@ -265,6 +265,25 @@ cdef class CodecContext(object):
                 break
         return out
 
+    def _send_packet_and_recv_reuse(self, Packet packet):
+
+        cdef Frame frame
+
+        cdef int res
+        with nogil:
+            res = lib.avcodec_send_packet(self.ptr, &packet.struct if packet is not None else NULL)
+        err_check(res)
+
+        while True:
+            frame = self._recv_frame()
+            if frame:
+                self._setup_decoded_frame(frame, packet)
+                yield frame
+                self._next_frame = frame
+            else:
+                break
+        return
+
     cdef _prepare_frames_for_encode(self, Frame frame):
         return [frame]
 
@@ -357,6 +376,24 @@ cdef class CodecContext(object):
             res.append(frame)
         return res
 
+    def xdecode(self, packet=None):
+        """Decode a list of :class:`.Frame` from the given :class:`.Packet`.
+
+        If the packet is None, the buffers will be flushed. This is useful if
+        you do not want the library to automatically re-order frames for you
+        (if they are encoded with a codec that has B-frames).
+
+        """
+
+        if not self.codec.ptr:
+            raise ValueError('cannot decode unknown codec')
+
+        self.open(strict=False)
+
+        for frame in self._send_packet_and_recv_reuse(packet):
+            yield frame
+        return
+
     cdef _setup_decoded_frame(self, Frame frame, Packet packet):
 
         # Propagate our manual times.

av/container/input.pyx

@@ -163,6 +163,23 @@ cdef class InputContainer(Container):
             for frame in packet.decode():
                 yield frame
 
+    def xdecode(self, *args, **kwargs):
+        """xdecode(streams=None, video=None, audio=None, subtitles=None, data=None)
+
+        Yields a series of :class:`.Frame` from the given set of streams::
+
+            for frame in container.decode():
+                # Do something with `frame`.
+
+        .. seealso:: :meth:`.StreamContainer.get` for the interpretation of
+            the arguments.
+
+        """
+        id(kwargs) # Avoid Cython bug; see demux().
+        for packet in self.demux(*args, **kwargs):
+            for frame in packet.xdecode():
+                yield frame
+
     def seek(self, offset, whence='time', backward=True, any_frame=False, stream=None):
         """Seek to a (key)frame nearsest to the given timestamp.
 

av/video/frame.pyx

@@ -96,7 +96,8 @@ cdef class VideoFrame(Frame):
 
     cdef _init_user_attributes(self):
         self.format = get_video_format(<lib.AVPixelFormat>self.ptr.format, self.ptr.width, self.ptr.height)
-        self._init_planes(VideoPlane)
+        if self.planes is None:
+            self._init_planes(VideoPlane)
 
     def __dealloc__(self):
         # The `self._buffer` member is only set if *we* allocated the buffer in `_init`,

av/packet.pyx

@@ -101,6 +101,13 @@ cdef class Packet(Buffer):
         """
         return self._stream.decode(self)
 
+    def xdecode(self):
+        """
+        Send the packet's data to the decoder and return a list of
+        :class:`.AudioFrame`, :class:`.VideoFrame` or :class:`.SubtitleSet`.
+        """
+        return self._stream.xdecode(self)
+
     @deprecation.method
     def decode_one(self):
         """