microui SDL3 example

[BemusedButBloodthirsty]

Checklist before submitting:

• This example has been added to .github/workflows/check.yml (for automatic testing)
• This example compiles cleanly with flags -vet -strict-style -vet-tabs -disallow-do -warnings-as-errors
• This example follows the core naming convention: https://github.com/odin-lang/Odin/wiki/Naming-Convention (exception can be made for ports of examples that need to match 1:1 to the original source).
• By submitting, I understand that this example is made available under these licenses: Public Domain and Odin's BSD-3 license. Only for third-party dependencies are other licenses allowed.