int64 CopyStream(Stream& dest, Stream& src, int64 count, Gate<int64, int64> progress)
{
	int block = (int)min<int64>(count, 32768);
	Buffer<byte> temp(block);
	int loaded;
	int64 done = 0;
	int64 total = count;
	while(count > 0 && (loaded = src.Get(~temp, (int)min<int64>(count, block))) > 0) {
		count -= loaded;
		done += loaded;
		if(progress(done, total))
			return -1;
		dest.Put(~temp, loaded);
	}
	return done;
}