Add SDL3 callback appmodel example.

[bplu4t2f]

Example of how to use the SDL3 callback based application model:

• sdl3.RunApp
• sdl3.EnterAppMainCallbacks

In a C program, this is normally done behind the scenes by SDL3 header files, so it may not be obvious how to set it up in Odin.

See also: https://wiki.libsdl.org/SDL3/NonstandardStartup

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 the naming and style convention: https://github.com/odin-lang/examples/wiki/Naming-and-style-convention (exceptions 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 zlib license. Only for third-party dependencies are other licenses allowed.