fix, new back-end utility and restoring the necessary functionality.

Eliminated some unnecessary functions that the refactor took care of,
linux build should be fixed.

Viewer added a new callback when the avatar returned without resetting the
timer for last reference to the saved raw image. This created a time window
in which new callbacks could get cleared out due to the raw image getting destroyed.

Since the callback was removed, pants flare was not properly applied. Appears
to be working now.

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)

SH-3455 WIP - moved setAvatarObject() to run after the self av constructor. Disabled sendAgentWearablesUpdate.

When wearing multiple layers of pants, the morph masks for all layers would be composited,
resulting in the smallest intersection of all layers. Instead we should only be applying the
top layer. This change will affect all consumers of the llappearance library.

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.