Avoid deadlocks by using lambdas for context lock (#2625)

ctx.input().key_pressed(Key::A) -> ctx.input(|i| i.key_pressed(Key::A))

examples/hello_world_par/README.md

@@ -0,0 +1,5 @@
+This example shows that you can use egui in parallel from multiple threads.
+
+```sh
+cargo run -p hello_world_par
+```