Make ImageLoader use background thread #13309

Job	Run time
android	30s
cargo-deny aarch64-apple-darwin	41s
Check wasm32 + wasm-bindgen	2m 19s
cargo-deny aarch64-linux-android	38s
cargo-deny i686-pc-windows-gnu	40s
cargo-deny i686-pc-windows-msvc	41s
Check wasm32+atomics	1m 50s
Format + check + test	2m 19s
cargo-deny i686-unknown-linux-gnu	38s
cargo-deny wasm32-unknown-unknown	40s
ios	23s
cargo-deny x86_64-apple-darwin	38s
Run tests	1m 31s
cargo-deny x86_64-pc-windows-gnu	39s
Check Windows	1m 23s
cargo-deny x86_64-pc-windows-msvc	38s
cargo-deny x86_64-unknown-linux-gnu	41s
cargo-deny x86_64-unknown-linux-musl	39s
cargo-deny x86_64-unknown-redox	45s
	18m 13s

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