Add OS thread locking in example code

[tenten8401]

If you're using this example code as the base for a project and start running other code in goroutines and getting the scheduler involved, the C bindings do not like this very much and the GUI will completely lock up when the scheduler automatically switches the imgui calls between OS threads.

[tinhajj]

I was noticing this today, struggling to figure out why my gui was locking up too. I tried the oslock like you suggested and that worked for me as well. Thanks