Bump imgui to 0.9

[aholtzma-am]

Checklist

• cargo clippy reports no issues
• cargo doc reports no issues
• cargo deny issues have been fixed or added to deny.toml
• human-readable change descriptions added to the changelog under the "Unreleased" heading.
  • If the change does not affect the user (or is a process change), preface the change with "Internal:"
  • Add credit to yourself for each change: Added new functionality @githubname.

Description

Bump imgui version to 0.9.0. Fix examples to match.

Related Issues

[aholtzma-am]

It's not clear to me how to solve the cargo-deny problems.

[cwfitzgerald]

I'll deal with that separately. Thank you!