Fix CVPixelBuffer/CMSampleBuffer ownership issues

src/platform/macos/nv12_zero_device.h

@@ -5,8 +5,13 @@
 #pragma once
 
 #include "src/platform/common.h"
+#include "src/platform/macos/av_img_t.h"
+
+struct AVFrame;
 
 namespace platf {
+  void
+  free_frame(AVFrame *frame);
 
   class nv12_zero_device: public avcodec_encode_device_t {
     // display holds a pointer to an av_video object. Since the namespaces of AVFoundation
@@ -27,6 +32,10 @@ namespace platf {
     convert(img_t &img);
     int
     set_frame(AVFrame *frame, AVBufferRef *hw_frames_ctx);
+
+  private:
+    util::safe_ptr<AVFrame, free_frame> av_frame;
+    av_img_t backing_img;
   };
 
 }  // namespace platf