Previously, the arithmetic assignment operators (`+=`, `-=`, `*=`, `/=`)
on `VecBaseCommon` only accepted another subclass of `VecBaseCommon`.
Changed the operators to accept any type.
The benefit of this change is that the assignment operator of classes
that inherit from `VecBaseCommon` can now work with `Vec`-like classes
that do not inherit from `VecBaseCommon`. I think the only real downside
is that the template could match other classes that would lead to
invalid comparisons. But such use would lead to a compile error anyway.
The difference is that instead of getting an error that no overloaded
version of the operator is available, you will get an error inside the
code of the operator.
1cc6dbb0c2