Geometric primitives for UI

## What problem does this solve or what need does it fill?

There are many cases where it would be useful to draw geometric primitives within a UI context:

* Lines or curves connecting nodes in a node graph.
* A "rubber-band selection rect" drawn with dashed lines.
* geometric shapes in a drawing app.

While Bevy has a rich set of geometric primitives, both 2d and 3d, they can't be used in a bevy_ui context but instead must be rendered on a separate layer. This means that you can't:

* Have primitives which are clipped to their parent.
* Have primitives which scroll when their parent scrolls.
* Have primitives that appear in front of some UI elements and behind others, other than by assigning a unique Z-index to every UI element.

The reason this doesn't work is because bevy_ui nodes require certain components, such as `Node` and `Style` which are not present on 2d primitives. Simply inserting these components doesn't work either, for example adding `Node` will cause the entity to be rendered as a box, even if it also has a `Mesh` component.

## What solution would you like?

Ideally, what I would like is a bundle (or whatever concept is going to replace bundles in the future). Just as we have various bevy_ui bundles for Node, ImageNode and so on, we'd have a `UiGeometryBundle` or something that would operate much like the current 2d primitives (with a `Mesh`, `Material` and so on), but would play nicely with bevy_ui.

## What alternative(s) have you considered?

I've tried a number of workarounds to this, but none are satisfactory. The best approach so far is an SDF-based UiMaterial that renders geometric primitives, but it has a bunch of drawbacks:

* It means having to render the entire bounding-box of the geometry, where most of the pixels are transparent.
* It means I can't re-use the existing Bevy 2d primitives, but instead have to come up with SDF equations for each type of geometry - that's a lot of tricky math.