LockedCamera/VideoReader subclass callback example

Create an example subclassing LockedCamera and/or VideoReader using `get_camera_image` as a callback within the main processing function, for processing which doesn't lend itself to being split in two, and/or variable processing times that can predict vaguely when it will finish and can more accurately try to have grabbing return precisely when processing completes.

首先閱讀 LockedCamera 和 VideoReader 的進入點，以及 get_camera_image callback 的使用方式。建立一個將處理保留在單一 main 函式中的範例，其中包含變動的處理時間和預期的擷取時序行為；該範例應能示範 callback 流程和時序策略。

由索引模型根據 Issue 內容生成。