Optimize Plot

Hi!

## Cloning

I'm trying to use the `Plot` and `Line` API.
This requires that I consume my data by value, I'd much rather keep a cached copy & provide a reference.

My plot data is fairly large (1M+ elements, sometimes 50M).

If this is not possible, can I add the API to accept something which can be `.as_ref()` to a `&[(f64, f64)]`?

I suspect the by-value nature of the API has something to do with the immediate mode design of `egui`... however, in cases like this it doesn't seem to actually make sense if it is driven only be design purity, ykwim? Your thoughts on this / advice would help me.

## Background Work

I am running massive simulations (~hours completion time) & interactively showing the results over time is the goal. I'd like to be able to do this in the background as mentioned. It is mentioned a few times in the documentation that long-running tasks should be deferred to background tasks in an async manner. I agree. I'm lost on how to accomplish this. I can think of a few clumsy ways, but I want to know if there is a proper way to do this that makes things easy and performant in `egui`.

Thank you!