Fix std::vector refactor mistake

[messmerd]

When usages of QVector were replaced with std::vector in the Core in #6477, some methods that now return const std::vector& were left unchanged at the call sites. In many cases, the const reference returned by these methods was stored by value instead of by const reference. Because the QVector objects are implicitly shared and have copy-on-write behavior, this previously didn't really matter and was a constant time operation when the objects were only read from - as was the case most of the time. But now with std::vector, all of these std::vector objects are being deep copied which is O(n) and is not real-time safe.

This PR fixes that oversight by using references at the call sites.

[sakertooth]

Were you able to verify that these were all the applicable call sites?

[messmerd]

Were you able to verify that these were all the applicable call sites?

I'm pretty sure I covered everything. I searched for every std::vector, type alias for a std::vector, every method returning or accepting a std::vector of some form, and every usage of those methods in include/ and src/.

[sakertooth]

LGTM, just a question.