Backend should respect idx and vtx offsets given by ImGui

[sarchar]

This fixes background dimming with modal popups and (probably) the CTRL+Tab box

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

Related Issues

[sarchar]

Looks like this might address #51