precision lose using Pos2 and Vec2

Hello

**Describe the bug**

I have a very big ScrollArea and want to place an element, let's say, in the middle of it

If I want to place my element at `345045100`

But if I want to place my next element at `345045100 + 15.125 = 345045115.125`

It will not work, because the f32 isn't precise enough

```rs
fn main() {
let a: f32 = 345045100_f32;
let b: f32 = 15.125_f32;
println!("{a} + {b} = {} (f32)", a + b);
println!("a.next_up() = {} (f32)", a.next_up());
let a: f64 = 345045100_f64;
let b: f64 = 15.125_f64;
println!("{a} + {b} = {} (f64)", a + b);
println!("a.next_up() = {} (f64)", (a as f64).next_up());
}
// 345045100 + 15.125 = 345045100 (f32)
// a.next_up() = 345045120 (f32)
// 345045100 + 15.125 = 345045115.125 (f64)
// a.next_up() = 345045100.00000006 (f64)
```

I think it would be nice to have Pos2 and Vec2 as f64 available !

Start by running the Rust precision reproduction from the issue and reviewing the existing Pos2 and Vec2 APIs. Determine the scope and compatibility requirements for making f64-based position and vector types available; done means large ScrollArea coordinates can represent the requested fractional offset accurately.

Written by the indexing model from the issue text.