Make ImageLoader use background thread #13311

Job	Run time
android	28s
cargo-deny aarch64-apple-darwin	38s
cargo-deny aarch64-linux-android	35s
cargo-deny i686-pc-windows-gnu	39s
Check wasm32 + wasm-bindgen	2m 28s
cargo-deny i686-pc-windows-msvc	38s
cargo-deny i686-unknown-linux-gnu	40s
cargo-deny wasm32-unknown-unknown	40s
Check wasm32+atomics	1m 23s
cargo-deny x86_64-apple-darwin	39s
cargo-deny x86_64-pc-windows-gnu	39s
Format + check + test	2m 29s
ios	21s
cargo-deny x86_64-pc-windows-msvc	38s
cargo-deny x86_64-unknown-linux-gnu	38s
Run tests	1m 38s
Check Windows	1m 18s
cargo-deny x86_64-unknown-linux-musl	39s
cargo-deny x86_64-unknown-redox	41s
	17m 49s

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