[3.x] Change Node children to use LocalVector

[lawnjelly]

There is no need for COW, it will only slow Node down.

LocalVectori (signed) rather than LocalVector

One gotcha when changing between Vector and LocalVector is that the default size for LocalVector is unsigned, and Vector is signed. Usually unsigned makes more sense for counters, but in this case, children is used in quite a lot of code in Node and in the interests of not introducing any new bugs, I have just changed it to the signed variation of LocalVector.

If we desire to change to unsigned LocalVector, that is probably best as a follow up, then modify the counters etc in Node.cpp.

Notes

• First commit is [3.x] Remove implicit conversion from LocalVector to Vector #107543, as we are using explicit LocalVector to detect any expensive conversions (there were none).
• Only 4 lines of code needed, may give some speedup as there in no COW machinery needed, and Node children are used a lot.
• As with Spatial children and CanvasItem children, really should have converted these when we created LocalVector, but hey, better late than never. 😁
• This is 3.x specific, 4.x uses different containers for storing Node children.

[Ivorforce]

I was about to comment that get_children will now need to create a copy, but it actually already does because it needs to return an Array. Makes sense 😅
As far as I can see the CoW property of children is never used, so LocalVector makes more sense.

Note that LocalVector never shrinks, as opposed to Vector, so this may start zombying memory. This may not be a problem because it's unlikely a Node will be used by adding thousands of children, only to shrink and never have them again. But it's good to keep in mind; we may want to use hysteresis shrinking in later iterations.

[lawnjelly]

Yup as discussed on RC, it is likely worth using hysteresis for Node, Spatial and CanvasItem children. 👍

[lawnjelly]

Thanks!