Add a "path strip" primitive for 2d and 3d.

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

This is a generalization of the "linestrip" concept which is currently supported in Gizmos, but not as a primitive. The use case I'm thinking about is a Blender-style shader editor with connecting wires, but there are potentially other use cases as well.

## What solution would you like?

What you would have is an array of SVG-like path elements:

```rust
#[derive(Debug, Copy, Clone)]
pub enum DrawablePathSegment {
/// Move to a new position.
Move(Vec2),
/// Draw a straight line to a new position.
Line(Vec2),
/// Draw a quadratic curve to a new position.
Quadratic((Vec2, Vec2)),
}
```
The primitive would convert this into line segments at some level of granularity so that the curves don't look jagged.

There should also be a "gradient" version, similar to `linestrip_gradient`.

## What alternative(s) have you considered?

There are several alternatives, but none of them completely satisfactory:
* Doing my own linestrips is possible, but that work should be shared.
* Gizmos are problematic in UI contexts since you can't control what UI layer they appear on.
* Using an SDF-based shader is also possible but not terribly efficient since you have to render the whole bounding box of the path.

Start by reading the existing Gizmos linestrip and linestrip_gradient implementations, then compare them with the proposed DrawablePathSegment API. Determine how a 2D and 3D primitive should represent Move, Line, and Quadratic segments and how curve subdivision granularity should work. Done means path strips render without jagged curves and include a gradient variant.

Written by the indexing model from the issue text.