Introduce ShaderModel. #753

Job	Run time
nix-fmt-check	22s
nix-flake-check (ubuntu-latest)	33s
cargo (cargo fmt --verbose --all -- --check)	58s
nix-flake-check (macos-latest)	55s
cargo (cargo check --locked --workspace)	4m 25s
cargo (cargo check --locked --examples --workspace)	3m 15s
cargo (cargo test --locked --lib --bins --workspace --features "egui")	9m 58s
cargo (cargo doc --locked --workspace --features "egui")	8m 25s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	1m 39s
cargo (mdbook build guide/)	1m 20s
cargo (cargo test --locked -p book_tests)	6m 26s
verifications	0s
cargo-publish	0s
guide-push-to-deploy	0s
	38m 16s

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