Implement Vector2 math needed by Brothers - A Tale of Two Sons launcher
For https://bugs.winehq.org/show_bug.cgi?id=57406
This also requires rewriting Vector2.get_Empty to have modopt(IsConstModifier), which is the first time we've had to do that for a method rather than a field.
Normalize may be missing some corner cases, like zero-length vectors, not sure if native handles those or not.