Record the file pointer position of each frame

def determine_frame_positions(self):
        """Record the file pointer position of each frame"""
        self.rewind_file()
        with ignored(struct.error):
            while True:
                pointer_position = self.blob_file.tell()
                length = struct.unpack('<i', self.blob_file.read(4))[0]
                self.blob_file.seek(length - 4, 1)
                self.frame_positions.append(pointer_position)
        self.rewind_file()
        log.info("Found {0} frames.".format(len(self.frame_positions)))