Blender Git Loki

Fix undefined behavior with --debug-xr

Mistake in cb578ca1048d3. Before that, the extension vector was static,
to make sure the extension name strings wouldn't get destructed when
leaving the function. I didn't think that was an issue and couldn't
recreate one, because until the previous commit we wouldn't actually
add any extensions to the vector on Windows (the system I tested
with).

Use C++17's `std::string_view` now, which avoids the string copies
`std::string` creates for itself and thus its destruction when leaving
the local scope.