2020-09-10: function-based fullscreen bouncing watermarks, a new metadata API, and BUG fixes

This update mainly adds a practical fullscreen bouncing watermark. Studying ffmpeg command-line functions showed they could achieve many desired effects, so I spent several days implementing it. The image watermark starts at top-center, moves to middle-right, bounces toward bottom-center, then middle-left and back to top-center:

It is integrated into the WYSIWYG editor, with configurable start time, interval and duration. Run it continuously, repeat a bounce cycle at intervals or delay its initial appearance. Image-watermark movement is considerably harder than an equivalent text effect; the movement function exceeds 300 characters. I hope you enjoy it.

Second, we replaced Douban's API, since its API endpoints were no longer available. The new API provides Chinese and English information plus Douban, IMDB and Rotten Tomatoes ratings. Details: https://www.efvcms.com/blog/5f5a2df813720e0d078fb92a.

Third, fixed BUG issues including missing posters in CMS popular videos and a BUG that stopped processing when administration settings requested only MP4 transcoding without slicing. Other code improvements increase stability.

Animated demonstration and screenshots