Share current frame capture on SNS

[lordfriend]

Although this is a private video system that will not encourage any share on public web, but there are requirement for sharing video frame capture and with some comment on SNS aka ツッコミ

Using an ffmpeg to take a frame capture can easily cover this requirement. So it is needed to add a new API for this function.

API Proposal

[lordfriend]

The feature is partially implemented because it is hard share image without sharing the link of site.
So the implementation is simple capture current frame and download this image when in Chrome / Firefox and Edge or show a preview in other browser.

Note that Safari has a bug on capture frame
https://bugs.webkit.org/show_bug.cgi?id=153588