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`