Fix for invisible avatar megaprim "featurebug" (archaic method of avatar body hiding)

**NOTE**: Please don't simply auto-merge this; inspect the aftermath and if preferred, move entries from settings_effervescence.xml into settings.xml

* Re-enabled fix for avatar body invisibility featurebug (archaic)
* Added option to adjust maximum attachment span with settings
* Added option to enable automatic derendering of huge attachments (use with care)