`mesa-3.x` legacy branch for Mesa 3.x examples

[EwoutH]

We've created a mesa-3.x branch for Mesa 3.x examples, while we continue development towards Mesa 4.0.

For new pull requests:

• To update/add an example for Mesa 4.x, target the main branch (default)
• To update/add an example for Mesa 3.x, target the mesa-3.x branch

Checkout our HISTORY.md for the latest release notes.