Fix material bug. #62

Job	Run time
nix-flake-check (ubuntu-latest)	20s
nix-fmt-check	20s
nix-flake-check (macos-latest)	1m 9s
cargo (cargo fmt --verbose --all -- --check)	21s
cargo (cargo check --locked --workspace)	21s
cargo (cargo check --locked --examples --workspace)	22s
cargo (cargo test --locked --lib --bins --all-features --workspace)	25s
cargo (cargo doc --locked --all-features --workspace)	22s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	36s
cargo (cargo build --locked -p nannou --features "wasm-experimental" --target wasm32-unknown-unkn...	24s
cargo (mdbook build guide/)	26s
cargo (cargo test --locked -p book_tests)	34s
verifications	0s
cargo-publish	0s
guide-push-to-deploy	0s
	5m 40s

Provide feedback