AssimpImporter: Support multiple meshes per node

[ikalevatykh]

If a node has several meshes, creates one Object3D per mesh.

object3DCount() query returns a number of all nodes extended with a number of extra nodes for each additional mesh.
object3DForName() points to the first object containing the first mesh,
object3DName() returns the same name for all objects in the given sequence.
doObject3D() for multi-meshes returns node for first mesh as usual but also returns indices of extra meshes nodes as children.

[mosra]

Wow, you were fast :)

So, if I see correctly, this is the same as I outlined in #66 (comment) (and as documented in TinyGltfImporter), right? If so, can you:

• add a similar "Mesh import" section to AssimpImporter.h like it was in TinyGltfImporter, so people know the semantics
• any chance you could add a test case for this? In an Ideal World™, the same glTF file as is used to test this exact thing in TinyGltfImporter could be used here (and the same test code as well, so basically copy the test part of da58734 over), but realistically you might hit a bunch of crash bugs with Assimp :/ Then the second easiest would be converting that glTF file to Collada somehow and then letting Assimp load that.

[mosra]

@ikalevatykh any chance you could have a look at the tests & docs? It would be great to have this in for the release, but I probably won't be able to find time to add those myself.

Thank you!

[ikalevatykh]

@mosra I had to switch to another task but I will do it Thursday

[mosra]

You're great, thanks!

[mosra]

Merged a variant of this in #81. Thanks for providing the initial version! 👍