Update Piet to v0.7.0-cairo18. #5795

Job	Run time
cargo clippy+test (gtk)	1m 10s
cargo clippy+test (windows-latest)	2m 0s
druid doctests	57s
Docs (windows-latest)	3m 44s
Docs (macos-latest)	1m 3s
mdbook build	22s
Docs (ubuntu-latest)	4m 6s
cargo fmt	21s
cargo clippy+test (macos-latest)	1m 27s
cargo clippy+test (ubuntu-latest)	1m 53s
cargo test (wasm32)	1m 33s
cargo clippy+test (wayland)	1m 24s
	20m 0s

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