Media: Thumbnails

[kean]

Part of #21457

I made a couple of misguided changes in #21562 when it comes to thumbnails. This PR reverts some of them but keeps the performance improvements and adds more. These changes are hard to test manually, so I added unit tests. I'll add more as I add "official" support for GIFs, videos, and other formats.

In #21562, I tried switching to media.absoluteThumbnailLocalURL for all thumbnails, but it turned out it's used for large thumbnails that can be used fullscreen. Using small thumbnails breaks layout in the editor: recording (under the feature flag only).

This PR has the following changes:

• ⚠️ This is the only (small) change that affects the production code. Set the maximum scale of large thumbnails mentioned earlier to 2x (3x is way too much for large thumbnails, and it looks like this code hasn't been updated for 3x phones). Example: 2796×1857 px (1.4 MB) → 1864 × 1238 px (542 KB). In addition to that, I changed the JPEG quality to an optional 80%, which reduces the size even further without any noticeable loss in quality: 542 KB → 406 KB. So, overall, it's a 3.5x improvement.
• Introduces a targetSize(forMediaSize,targetSize) method to make sure this doesn't happen: recording. It's also a prerequisite for adding an "aspect" mode to the photos, like in the native Photos app.
• Use 1x scale for GIFs. I'll make them animated in the future PRs, and it'll look good. There is a precedent in the app for doing that. It results in a massive size reduction, example, 2.4 MB → 373 KB (6.5x improvement), and with more than acceptable quality for an animated GIF.
• Switch to magic numbers to identify image format and determine when decompression is needed (more reliable than file extensions)
• Add a migration step that clears all previously cached thumbnails, as most of them will need to be re-fetched anyway with a more efficient format and/or size when the feature goes out

I'm not sure I like the use of @MainActor, but other than that, I think it's almost there now. Maybe a few minor tweaks. I'll likely move all of this work to the background in the next PR, but I need to figure out how to do it efficiently with Core Data.

To Test

It's hard to test it manually, so I ended up writing a set of unit tests. Instead of duplicating all the test scenarios here, please follow the unit tests.

I asked not to test in the previous PRs because of the imminent upcoming changes. Now is the time to do some testing, preferably with a debugger.

Regression Notes

1. Potential unintended areas of impact: Post Editor (add photo)
2. What I did to test those areas of impact (or what existing automated tests I relied on): manual
3. What automated tests I added (or what prevented me from doing so): yes

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist:

• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn't complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr21615-8e553ec
	Version	23.3
	Bundle ID	org.wordpress.alpha
	Commit	8e553ec
	App Center Build	WPiOS - One-Offs #7177

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr21615-8e553ec
	Version	23.3
	Bundle ID	com.jetpack.alpha
	Commit	8e553ec
	App Center Build	jetpack-installable-builds #6218

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[momo-ozawa]

Confirmed reported regressions are resolved. Followed unit tests - works as described!