egui_glium: run app code outside event loop to fix file dialogs (#631)

Previously app code was run from within the event loop
which lead to file dialogs (e.g. using nfd2) to hang
(see https://github.com/rust-windowing/winit/issues/1779)

Now egui_glium polls for events and then runs the app code.

eframe/CHANGELOG.md

@@ -4,6 +4,7 @@ All notable changes to the `eframe` crate.
 
 ## Unreleased
 * Improve http fetch API.
+* `run_native` now returns when the app is closed.
 
 
 ## 0.13.1 - 2021-06-24