feat(core): Add factory destroy policy 'never' (9.1)

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For #

Background

• We have implemented caching via the Pipeline and Shader Factory classes.
• This means that if the app is creating several identical such objects at the same time, we will only create one copy.
• However we destroy cached elements as soon as they are no longer referenced.
• An app that repeatedly creates and destroys an object (perhaps a temporary layer or transform) won't benefit.
• While the perfect model would probably be a system that held on to only a certain number of released objects on an LRU-basis, that would require more book-keeping.
• This. PR adds an experimental Device option _factoryDestroyPolicy: 'never' that simply stops destroying unused objects and keeps them in the cache.
• Intended to allow us to test whether longer life of cached items has sufficient performance advantages to be worthwhile.

Change List

• experimental Device option _factoryDestroyPolicy: 'never'
• Factory classes copy the option and avoid destroying object if it is set.

[ibgreen]

This line with the new option is the only significant change in this file, the rest was reorganization for readability as number of options grows.