Add external eventloop support (#6750)

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* Closes #2875
* Closes https://github.com/emilk/egui/pull/3340
* [x] I have followed the instructions in the PR template

Adds `create_native`. Similiar to `run_native` but it returns an
`EframeWinitApplication` which is a `winit::ApplicationHandler`. This
can be run on your own event loop. A helper fn `pump_eframe_app` is
provided to pump the event loop and get the control flow state back.

I have been using this approach for a few months.

---------

Co-authored-by: Will Brown <opensource@rebeagle.com>

examples/external_eventloop/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "external_eventloop"
+version = "0.1.0"
+authors = ["Will Brown <opensource@rebeagle.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.84"
+publish = false
+
+[lints]
+workspace = true
+
+
+[dependencies]
+eframe = { workspace = true, features = [
+    "default",
+    "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
+] }
+
+env_logger = { version = "0.10", default-features = false, features = [
+    "auto-color",
+    "humantime",
+] }
+
+winit = { workspace = true }