Preliminary Scrubbing

[EricBAndrews]

Issues

• closes GIF/Video Scrubbing #173

Description

This PR lays out the core architecture for gif/video scrubbing. MediaControlState now tracks two new fields:

• playbackPosition: set by the animated media; indicates current playback position
• scrubTarget: set by the control layer; informs the animated media where to scrub to

The underlying animated media types have been updated to populate and listen to those fields, respectively.

There are a few outstanding issues that will be addressed in later PRs:

• UI: this PR comes with a rudimentary playback bar. The circle can be dragged to scrub. It's not very pretty and a bit of a pain to use, but the point of it is just to provide a minimal working interface to scrub videos. Improved GIF/Video Scrubbing #1834 is open to track this.
• Performance when scrubbing backwards: all media types, but webp and gif in particular, suffer when scrubbing backwards. There are some tricks that might help performance here, but most of them will require rewriting AnimatedImageView using UIViewRepresentable around the underlying SDWebImage UIKit components. Better webp, gif rendering #1883 is open to track this.

Implementation Notes

• Removes Gifu. SDWebImage now handles all animated images. The .gif and .webp cases of the MediaType enum have been merged into .animated.

[Sjmarf]

Awesome work! A couple of localization notes