Fix CVPixelBuffer/CMSampleBuffer ownership issues

src/platform/macos/av_img_t.h

@@ -10,10 +10,43 @@
 #include <CoreVideo/CoreVideo.h>
 
 namespace platf {
-  struct av_img_t: public img_t {
-    CVPixelBufferRef pixel_buffer = nullptr;
-    CMSampleBufferRef sample_buffer = nullptr;
+  struct av_sample_buf_t {
+    av_sample_buf_t(CMSampleBufferRef buf):
+        buf((CMSampleBufferRef) CFRetain(buf)) {}
 
-    ~av_img_t();
+    ~av_sample_buf_t() {
+      CFRelease(buf);
+    }
+
+    CMSampleBufferRef buf;
+  };
+
+  struct av_pixel_buf_t {
+    av_pixel_buf_t(CVPixelBufferRef buf):
+        buf((CVPixelBufferRef) CFRetain(buf)),
+        locked(false) {}
+
+    uint8_t *
+    lock() {
+      if (!locked) {
+        CVPixelBufferLockBaseAddress(buf, kCVPixelBufferLock_ReadOnly);
+      }
+      return (uint8_t *) CVPixelBufferGetBaseAddress(buf);
+    }
+
+    ~av_pixel_buf_t() {
+      if (locked) {
+        CVPixelBufferUnlockBaseAddress(buf, kCVPixelBufferLock_ReadOnly);
+      }
+      CFRelease(buf);
+    }
+
+    CVPixelBufferRef buf;
+    bool locked;
+  };
+
+  struct av_img_t: public img_t {
+    std::shared_ptr<av_sample_buf_t> sample_buffer;
+    std::shared_ptr<av_pixel_buf_t> pixel_buffer;
   };
 }  // namespace platf