internal symbol clones
Rendering symbols is a NON-CONST operation, which can permanently
alter the symbol instance (e.g. via changes made by the symbol
or symbol layer's startRender methods.).
This makes debugging super complex - because methods which look
like they are just generating previews of symbols can actually
change the original symbol instances, resulting in permanent changes
to a layer's style.
Refs #19910 (specifically, me pulling my hair out trying to deduce
seemingly random changes to layer's symbols)