Add example using three-d

https://github.com/asny/three-d recently merged a PR adding
`glow` support: asny/three-d#210
This means it is a prime candidate for embedding 3D painting inside
an eframe app.

There are currently a few kinks that need to be figured out:

When reusing the same three_d context over time (as one should),
we only get one frame of egui together with three_d, and then after that
a black screen with just the three_d painting on top.

I need to fix that before merging this PR.

`Shape` is `Send + Sync` and `three_d::Context` is not. This means
we cannot store a three_d context and send it to the `Shape::Callback`.

So we either need to recreate the three_d context each frame (obviously
a bad idea), or access it through a `thread_local` hack.
This PR adds both as examples, with a checkbox to switch.

We could consider making `Shape: !Send + !Sync`, but that would mean
`egui::Context` could not be `Send+Sync` either (because the egui
context stores shapes). This could actually be fine. `egui::Context`
should only be used from a background thread for calling request_repaint
and allocating textures. These could be made separate parts of the
egui Context, so that one would do:

``` rust

let repaint_signal = egui_ctx.repaint_signal();
let tex_mngr = egui_ctx.tex_mngr();
std::thread::spawn(move || {
    // We can use repaint_signal and tex_mngr here,
    // but NOT `egui_ctx`.
}):

Related:
* #1379
* #1380
* #1389