MAINT-3320 : Fixed! Inventory panels are not resistant to folder root deletion, using handle instead of pointer and cleaning things consistently

MAINT-3320 : WIP : Use handle for elements that'll get deleted under us, quite a bit of debug spam to clean later

MAINT-3270 FIXED crash in LLControlCache<unsigned int>::LLControlCache: Control named RenderAutoMuteFunctionsnot found

MAINT-3193 FIXED crash with LLControlCache<bool>::LLControlCache: Control named RenderAutoMuteLoggingnot found.

BUG-3982 add code to subtract the lack of not removing the removal of backfaces for particles (make particles render double-sided again)

MAINT-3179 fix bad indexing of position info generating false alarms on range checks, issue with free of data now allocated contig with positions instead, and added copious amounts of llwarns to error return sites so 'invalid element' will be a reason to read the log and not a tease.