SceneData field specification optimizations

[mosra]

Originally meant to be a part of #525, but at the very last second I realized nasty interactions with the compatibility single-function-object conversion tools, which have to get fixed first. Things to do:

• make SceneTools::combine() aware of implicit null mappings and trivial fields
  • not choking on them
  • probably also preserving them where possible instead of expanding
• SceneTools::convertToSingleFunctionObjects() needs to be aware of these as well
  • in particular not removing SceneFieldFlag::ImplicitMapping for fields passed to combine(), as those will turn implicit null fields into placeholders
  • should it instead unconditionally expand them? probably for the best, especially given that neither the parent field nor the implicit mapping will stay implicit or trivial

Looks like I dug a really deep pit and then managed to fall at the very bottom. Yey. Or maybe there's some other solution / different way of implementing implicit or trivial fields without making them null? Some possibilities:

• Make it possible to have each field in a separate allocation (and thus per-field DataFlags, etc.)?
  • thus the implicit / trivial fields could be a static memory / allocated in addition to a memory-mapped file
  • still won't solve the unnecessary overhead of having to allocate e.g. 400 kB of memory that contains just numbers 0-100k
  • and will make mapping shared between fields a lot more complicated, leading to each having to allocate its own (sigh!)
  • OTOH could make it easier to capture live data (and allow them to individually grow/shrink...)