FFMPEG streaming option for VideoWriter

Could be useful to allow users with ffmpeg installed to use that as a sub-processed backend instead of opencv's cv2.VideoWriter (as at https://stackoverflow.com/a/42602576, (and https://stackoverflow.com/a/13298538)). If doing, make a similar interface to existing VideoWriter class for consistency and ease of use.

cv2.VideoWriter works but is quirky, and generally ends up with large file sizes even when just duplicating existing videos (likely due to decompression on read that's not recompressed on write).

未指定任何檔案、測試或進入點；請先定位現有的 VideoWriter 類別及其與 cv2.VideoWriter 的整合。將提議的 ffmpeg 子程序方案與該介面進行比較，並在基於 ffmpeg 的寫入能夠提供一致的 VideoWriter 使用方式且避免所述的檔案大小問題時，視為工作完成。

由索引模型根據 Issue 內容生成。