User facing FFMPEG configuration

[StashAppDev]

stashapp/StashServer#41

[WithoutPants]

stash currently generates various images and videos with the following settings:
Screenshots:

• time location: (0.2 * duration)
• quality: 2
• width: video width
• output format: jpg
• exposed via /screenshot

Thumbnails:

• time location: (0.2 * duration)
• quality: 5
• width: 320
• output format: jpg

Note: thumbnails are currently not used by anything

Preview videos:

• video chunks: 12
• width: 640
• chunk duration: 0.75s
• output format: mp4 (x264/aac)
• exposed via /preview

Preview video webp:

• width: 640
• fps: 12
• output format: webp
• exposed via /webp

Scene marker videos:

• width: 640
• duration: 20s
• output format: mp4 (x264/aac)
• exposed via /stream

Scene marker images:

• width: 640
• duration 5s
• output format: webp
• exposed via /preview

Transcode videos:

• width: original width
• output format: mp4 (x264/aac)

Streaming transcodes

• width: original width
• output format: webm

[WithoutPants]

With #178 sorting out the transcode sizes, do we think there's anything else needed for this issue, or can it be closed?

[Leopere]

This was requested to be addressed on November 17th 2019 I'll close this pending any further commentary.