Fix broken variant call of Vector3.snapped

[clktmr]

No description provided.

[vnen]

This is broken since the beginning of time...

[hpvb]

Is there any way that the previous behavior did something sensible (if undocumented?) this makes me a little nervous for cherry picking into 3.0

[vnen]

@hpvb in my tests it just returned the same vector.

[akien-mga]

This is broken since the beginning of time...

Actually no, only since June 2017: 44ecfb0.

Vector3.snapped used to use a float as parameter, but the bindings weren't updated properly. We need to be extra careful with Variant changes, most contributors have no idea about how they're bound and thus miss some stuff.

[hpvb]

As this changes api/abi I don't think this should go into 3.0, it may break existing gdnative modules