We won't be able to retain render phases from frame to frame if the keys
are unstable. It's not as simple as simply keying off the main world
entity, however, because some main world entities extract to multiple
render world entities. For example, directional lights extract to
multiple shadow cascades, and point lights extract to one view per
cubemap face. Therefore, we key off a new type, `RetainedViewEntity`,
which contains the main entity plus a *subview ID*.
This is part of the preparation for retained bins.
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Co-authored-by: ickshonpe <david.curthoys@googlemail.com>
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