Optimize unloading of prims #2462
Description
Optimize LLMeshRepository::unregisterMesh.
LLViewerObjectList::cleanDeadObjects takes a long time to run, especially during shutdown, but also when moving around the world, leading to intermittent slow frames. LLMeshRepository::unregisterMesh was found to be a bottleneck for this function.
Test plan
Due to the nature of the performance fix, regressions are not expected. There is a small chance of increased crashes with LLMeshRepository in the stack trace, most noticably on shutdown.