Fix headless asset preprocessing hang (#49)

## Summary
- register Bevy's image loader explicitly in the headless asset processor
- use no GPU-compressed image formats because the processor has no render device
- allow PNG processing tasks to complete so `ProcessorState` reaches `Finished`

Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/bin/process-assets.rs

@@ -2,7 +2,7 @@ use bevy::{
     app::ScheduleRunnerPlugin,
     asset::{AssetMode, AssetPlugin, processor::AssetProcessor, processor::ProcessorState},
     gltf::GltfPlugin,
-    image::ImagePlugin,
+    image::{CompressedImageFormats, ImageLoader, ImagePlugin},
     prelude::*,
     tasks::block_on,
     text::TextPlugin,
@@ -19,6 +19,7 @@ fn main() -> ExitCode {
             ..default()
         })
         .add_plugins((ImagePlugin::default(), TextPlugin, GltfPlugin::default()))
+        .register_asset_loader(ImageLoader::new(CompressedImageFormats::NONE))
         .add_plugins(AssetProcessingPlugin)
         .add_systems(Update, finish_when_processed);