Add GUI resize support

[crazymykl]

Also, refactor the GUI to be able to integration test the event loop. We use nextest and a custom harness using libtest-mimic to make sure these examples run on the main thread, and each example gets its own process.

This was a bit of a yak shave.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1955037) to head (7cf7cfb).
Report is 6 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main        #9      +/-   ##
===========================================
+ Coverage   94.01%   100.00%   +5.98%     
===========================================
  Files           4         5       +1     
  Lines         451       673     +222     
  Branches      451       673     +222     
===========================================
+ Hits          424       673     +249     
+ Misses         22         0      -22     
+ Partials        5         0       -5     

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