Some sample applications do not exit cleanly

[bjoeris]

Some sample applications (e.g. timeline_semaphore_simple) crash on exit. It looks like part of what is going on is that the main loop has no exit condition, but changing this to match other samples is insufficient (e.g. I tried to change the while (true) to while (!app.should_exit() && !data->WindowClosing()) in "application_sandbox/timeline_semaphore_simple/main.cpp", but this application still crashes, so more investigation is needed.

Just based on the presence of a main loop that never terminates, I think this may impact the following samples (I have not verified that all of these actually crash):

• external_buffer
• external_image
• foreign_buffer
• present_region
• synchronization2_timeline_semaphore_simple
• timeline_sempahore_cross_queue
• timeline_semaphore_host_signal_after_submit
• timeline_semaphore_simple