Backend should respect idx and vtx offsets given by ImGui#110

This fixes background dimming with modal popups and (probably) the CTRL+Tab box <!-- Thank you for making a pull request! Below are the recommended steps to validate that your PR will pass CI --> ## Checklist - [ ] `cargo clippy` reports no issues - [ ] `cargo doc` reports no issues - [ ] [`cargo deny`](https://github.com/EmbarkStudios/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 ## Related Issues