VideoCommon: optimize asset loading system

[iwubcode]

This PR updates the asset loading system to optimize it. A brief overview is given below.

The old system

• Used a recursive_mutex, this seems to have had a chance to cause a crash on some MacOS machines
• Stuck everything into a single queue, loading everything after the other, users would report up to 30 seconds of wait time for initial texture loads with prefetch turned on. I could never replicate that but even on my modern machine, I would see up to 14 seconds for the textures I was interested in when toggling the load textures from a scene
• To handle realtime reloads of assets (for dynamic-input-textures [DIT] or manual asset edits), a separate thread existed that would peg the system every 500ms, looping over all assets to see if any were updated (this could have been a huge part of the performance bottleneck on bulk loading textures?)

The new system

• Assets can now be loaded in parallel by multiple threads. I currently have 2 threads defined but a future PR will likely create a setting to make this customizable
• There is a priority system. The system now tracks the last frame count for when an asset is used, assets with more recent frame counts are preferred over earlier ones
• I now use a library watcher to get told by the filesystem that an asset has changed. This marks the asset for reload. Avoiding the overhead of polling every asset for changes
• Assets can now be manually requested to be reloaded (used in my editor)

Testing

I have tested on Windows (11) both DIT and texture pack loads. The 'watcher' library has a couple of minor Windows bugs that I am working with the author to iron out but they shouldn't block this feature. (EDIT: Done) I need testing from other platforms (Android, Mac, Linux) to confirm all functionality operates and to get feedback from users with lower end system.

Windows DIT: Working
Windows Performance Experience: Positive, noticeably faster
MacOS DIT: Working
MacOS Performance Experience: --
Linux DIT: --
Linux Performance Experience: --
Android DIT: --
Android Performance Experience: --

[Tilka]

Not sure if relevant but we use QFileSystemWatcher for the game list. I guess it makes sense not to depend on Qt for this.