following promotion of DRTVWR-507

SL-14183, SL-14142 - impostor management improvements, animesh attachments update on frames when their parent does

LLHUDObject has a lot of deriveds stored as LLPointer<LLHUDObject>, no-virtual destructor is unlikely to be cause of the crash, but might be related

# Conflicts:
#	indra/newview/llfloatermodelpreview.cpp

following promotion of DRTVWR-482

LLVOAvatars and derived classes appear to be stored as LLPointer<LLViewerObject> in a lot of cases, so non-virtual ~LLViewerObject() might be triggered by LLPointer and end up not cleaning sInstances in some cases (animeshes specific?), thus causing broken pointers in sInstances.

SL-14045 - add a distinguishing flag in animation info so playing animations can be distinguished from others