Performance improvements for manual updates

[diarmidmackenzie]

Been working on auto-update mode & performance.

Remaining perf issue with manual updates - each manual update currently runs updateMatrixWorld() on the parent, and computes the inverse of the parent matrixWorld.

It would improve performance if we could calculate these once for multiple updates. One approach would be to batch mesh member position updates together, and do them in bulk on the tick, as we do with auto updates.

We already have a queuing mechanism for these updates, so not too much work.

Didn't do this as part of the auto-updates / perf work on the basis that we now have auto updates, and can always switch to that update mode if there's so many mesh member transform updates as to cause a performance issue in manual mode.

In other words:

• if a use case has few updates, manual update perf shouldn't be a big deal
• and if it has lots, then auto updates is probably a better solution anyway...

Not clear at this point that there's any use cases in between that would provide motivation for this fix.

[diarmidmackenzie]

Fixed by #25