[Type] Vec: Short-circuit toVecN for same-type conversions

[fredroy]

Skip conversion logic when both Vec types are the same. The compiler likely optimizes this already, but the explicit check makes the intent clearer and avoids relying on the optimizer to see through std::copy/std::fill_n.

Measured with alxbilger/SofaBenchmark#46
No change with -O3 but it should make lesser optimization/debug run faster

[with-all-tests]

---

By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

---

Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).