Render events

[ecoskey]

Objective

fix #18491 (comment)
Sometimes users want to send events from the render world to the main world. While not often advised, some advanced use-cases like readback use a similar pattern, which often requires setting up a channel and relaying events manually.

Solution

Add a simple abstraction (App::init_render_event) to set up the channel and relay events to the main world, retaining caller information.

Note: this pr makes Events::send_with_caller public, but does not add a similar public method on EventWriter

Testing

Ran example

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[JMS55]

Quickly leaving my thoughts, I don't love the events naming.

It has nothing to do with events about rendering, but is just an arbitrary channel for sending data back to the main world, right?

Not sure what a better name is though, and again haven't looked at the PR to deeply yet.

[IceSentry]

They are events from the render world. I think it's fair to call them render event. Presumably, users will need this to send events in the context of rendering. Otherwise they wouldn't need events sent from the render world. Maybe specify render world event just to remove any possible ambiguity, but I'm not sure what kind of ambiguity there could be.