Introduce ShaderModel. #752

Job	Run time
nix-flake-check (ubuntu-latest)	25s
nix-fmt-check	22s
nix-flake-check (macos-latest)	1m 0s
cargo (cargo fmt --verbose --all -- --check)	1m 0s
cargo (cargo check --locked --workspace)	4m 22s
cargo (cargo check --locked --examples --workspace)	4m 54s
cargo (cargo test --locked --lib --bins --workspace --features "egui")	10m 13s
cargo (cargo doc --locked --workspace --features "egui")	8m 16s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	1m 16s
cargo (mdbook build guide/)	1m 16s
cargo (cargo test --locked -p book_tests)	7m 25s
verifications	0s
cargo-publish	0s
guide-push-to-deploy	0s
	40m 29s

Provide feedback