Allow configuration of ffmpeg args (#3216)

* Allow configuration of ffmpeg args
* Add UI settings for ffmpeg config
* Add changelog entry
* Add documentation in manual

ui/v2.5/src/docs/en/Changelog/v0190.md

@@ -2,6 +2,7 @@
 * Performer autotagging does not currently match on performer aliases. This will be addressed when finer control over the matching is implemented.
 
 ### ✨ New Features
+* Added support for injecting arguments into `ffmpeg` during generation and live-transcoding. ([#3216](https://github.com/stashapp/stash/pull/3216))
 * Added URL and Date fields to Images. ([#3015](https://github.com/stashapp/stash/pull/3015))
 * Added support for plugins to add injected CSS and Javascript to the UI. ([#3195](https://github.com/stashapp/stash/pull/3195))
 * Added disambiguation field to Performers, to differentiate between performers with the same name. ([#3113](https://github.com/stashapp/stash/pull/3113))

ui/v2.5/src/docs/en/Manual/Configuration.md

@@ -77,6 +77,14 @@ This setting can be used to increase/decrease overall CPU utilisation in two sce
 
 Note: If this is set too high it will decrease overall performance and causes failures (out of memory).
 
+## ffmpeg arguments
+
+Additional arguments can be injected into ffmpeg when generating previews and sprites, and when live-transcoding videos. 
+
+The ffmpeg arguments configuration is split into `Input` and `Output` arguments. Input arguments are injected before the input file argument, and output arguments are injected before the output file argument.
+
+Arguments are accepted as a list of strings. Each string is a separate argument. For example, a single argument of `-foo bar` would be treated as a single argument `"-foo bar"`. The correct way to pass this argument would be to split it into two separate arguments: `"-foo", "bar"`.
+
 ## Scraping
 
 ### User Agent string