gpui: Optimize coalesce float sign checking (#28072)

Optimize away a multiplication during in the `coalesce` function. Our
goal is to check whether the sign of two floats is the same.

Instead of multiplying each `.signum()` and checking that the result is
positive, we can simply check that the signum's are the same. This
removes a float multiplication.

```rust
a.signum() * b.signum() >= 0.0
```

turns into

```rust
a.signum() == b.signum()
```



Release Notes:

- Fix documentation for `Pixels::signum`

crates/gpui/src/geometry.rs

@@ -2622,7 +2622,6 @@ impl Pixels {
     /// Returns:
     /// * `1.0` if the value is positive
     /// * `-1.0` if the value is negative
-    /// * `0.0` if the value is zero
     pub fn signum(&self) -> f32 {
         self.0.signum()
     }