Bump minimum e-g version to 0.7.1

[jamwaffles]

Thank you for helping out with embedded-graphics-simulator development! Please:

• Check that you've added passing tests and documentation
• Add an example where applicable
• Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, etc) if your changes affect the public API
• Run rustfmt on the project
• Run just build (Linux/macOS only) and make sure it passes. If you use Windows, check that CI passes once you've opened the PR.

PR description

Supersedes #32

[rfuest]

LGTM

[rfuest]

I haven't thoroughly checked this, but tinytga and tinybmp might also need to be updated to e-g 0.7.1. At least DynamicTga and DynamicBmp use the From color conversions.

[jamwaffles]

Noted, I'll get some PRs ready for them too in that case. It can't hurt to be cautious even if it's not required.