perf: use copysign over `if y < 0.0 { -x } else { x }`

[michaelkirk]

There are a few places in the codebase that do things like this.

-            t3 += if t3 < 0.0 { -disc.sqrt() } else { disc.sqrt() };
+            t3 += disc.sqrt().copysign(t3);

Searching for < 0.0 might lead you to some other examples.

It's a small thing but might help with branch prediction.