Add asset_processor feature and remove AssetMode::ProcessedDev

[cart]

Objective

Users shouldn't need to change their source code between "development workflows" and "releasing". Currently, Bevy Asset V2 has two "processed" asset modes Processed (assumes assets are already processed) and ProcessedDev (starts an asset processor and processes assets). This means that the mode must be changed in code when switching from "app dev" to "release". Very suboptimal.

We have already removed "runtime opt-in" for hot-reloading. Enabling the file_watcher feature automatically enables file watching in code. This means deploying a game (without hot reloading enabled) just means calling cargo build --release instead of cargo run --features bevy/file_watcher.

We should adopt this pattern for asset processing.

Solution

This adds the asset_processor feature, which will start the AssetProcessor when an AssetPlugin runs in AssetMode::Processed.

The "asset processing workflow" is now:

1. Enable AssetMode::Processed on AssetPlugin
2. When developing, run with the asset_processor and file_watcher features
3. When releasing, build without these features.

The AssetMode::ProcessedDev mode has been removed.

[cart]

In the future we can consider gating the AssetProcessor code behind this feature (in the interest of slimming down the binary), but I believe that isn't trivial with the current module structure.

[cart]

I think we should ultimately add one or more "full recommended dev experience" meta-features for simplicity and ergonomics. Ex: dev would enable file_watcher, embedded_watcher, asset_processor, tracing features, etc.

I think the dev experience is still "in flux" enough that we should wait a bit before defining these.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[alice-i-cecile]

Changes seem fine. Should asset_processor be on by default? I'd like to think we should generally prioritize new users for default features, rather than shipped games (which will be more deliberate).

[cart]

Should asset_processor be on by default? I'd like to think we should generally prioritize new users for default features, rather than shipped games (which will be more deliberate).

If it is on by default, it means that shipping would require disabling default features, which opens a huge can of worms that most users won't be equipped to manage. We aren't really even equipped to manage that, given the frequency of regressions we experience with non-default cargo feature configurations.

I think we should encourage the dev workflow as opt-in, as it is a cleaner approach, and also a better mental model (ex: please add in the "dev" features). Once we have the Bevy Editor we can hide this from most users with our "Play Button".

Additionally, I think we shouldn't yet encourage most people to turn asset preprocessing on by default. So far we've only built the image processor and it is pretty bare bones. Idk if the value prop is fully there yet for most people.