Add `compute` app function for running compute shaders #760

Job	Run time
nix-flake-check (ubuntu-latest)	26s
nix-fmt-check	27s
cargo (cargo fmt --verbose --all -- --check)	1m 18s
nix-flake-check (macos-latest)	58s
cargo (cargo check --locked --workspace)	4m 27s
cargo (cargo check --locked --examples --workspace)	3m 33s
cargo (cargo test --locked --lib --bins --workspace --features "egui")	10m 44s
cargo (cargo doc --locked --workspace --features "egui")	8m 32s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	1m 20s
cargo (mdbook build guide/)	1m 25s
cargo (cargo test --locked -p book_tests)	7m 39s
verifications	0s
guide-push-to-deploy	0s
cargo-publish	0s
	40m 49s

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