Added `forEachNodeInScene`

[lilleyse]

I found myself needing a function like forEachPrimitiveInScene but without drilling into meshes / primitives and just traversing over nodes.

[kring]

Thanks @lilleyse! Some of this code is pretty old, written before we had really settled on best practices for integer conversions. So I snuck in some more changes:

• Replaced int with int32_t.
• Use constructor notation for very safe integer conversions (where the range is checked) rather than static_cast.
• Use a better form of index checking. For example, rather than:

if (meshId >= 0 && meshId < static_cast<int32_t>(model.meshes.size()))

Do this:

if (meshId >= 0 && size_t(meshId) < model.meshes.size())

The former could reject valid indices in the (admittedly very unlikely) event that a model has more than 2^31 - 1 meshes.