[Feature] Add Size Extensions

## Describe the problem this feature would solve
We have a lot of this pattern in the toolkit:

```cs
new Rect(0, 0, size.Width, size.Height)
```

## Describe the solution
It'd be great to have some extensions for size:

```cs
size.ToRect() // Defaults to 0, 0
size.ToRect(point);
size.ToRect(x, y);
```

Thoughts?