Re-ordering code for SSS:
Now, SSS only is active with right integrators (DL,PM and PT)
Fix error "Plugin(s) left Garbage" with Area light editor.
Start review light export code wen separate background types.
Delete unused "warn_multiple_backgrounds" function.
Applied "freeze" patch from Micheus user.
Delete other unused code for improve readability and cleanup.
Separate "export_Background" code to other file( this code not work a.t.m.)