Bump pango, cairo to v0.18

[flxzt]

A new pango and cairo release is out, the changelog can be found here. There are no changes in the API that are used by piet as far as I can tell.

Can there be a new piet release at some point? The last one is using cairo v0.16 .

[xStrom]

I can merge if you also solve the unrelated clippy errors.

Error:    --> piet-web/src/lib.rs:547:13
    |
547 |         let mut canvas_state = self.canvas_states.last_mut().unwrap();
    |             ----^^^^^^^^^^^^
    |             |
    |             help: remove this `mut`

[xStrom]

Thanks!

As for making a release, it certainly makes sense to do a new release at some point this year. Making it just for this dependency update seems bit of an overkill, so I'll see if there's a few other PRs that could be merged as well.