video: Don't leak the gst::Buffer when converting a VideoFrame into an ffi::GstVideoFrame

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1220>

gstreamer-video/src/video_frame.rs

@@ -283,7 +283,11 @@ impl<T> VideoFrame<T> {
 
     #[inline]
     pub fn into_raw(self) -> ffi::GstVideoFrame {
-        mem::ManuallyDrop::new(self).frame
+        unsafe {
+            let mut s = mem::ManuallyDrop::new(self);
+            ptr::drop_in_place(&mut s.buffer);
+            s.frame
+        }
     }
 }