respect should_exit on linux as well

src/main_linux.cpp

@@ -452,7 +452,7 @@ int main(int, char**)
     init();
 
     // Main loop
-    while (!glfwWindowShouldClose(window))
+    while (!glfwWindowShouldClose(window) && !should_exit)
     {
         // Poll and handle events (inputs, window resize, etc.)
         // You can read the io.WantCaptureMouse, io.WantCaptureKeyboard flags to tell if dear imgui wants to use your inputs.