Add point cloud support to GLTFSceneKit

[chetan-set]

Hello,

Currently, GLTFSceneKit only supports the .triangles primitive. Consequently it is not possible to render point clouds using GLTFSceneKit. The change needed for that support is relatively minor and only affects one file. Furthermore, the change is quite isolated because to enter the point cloud code path, there is an explicit:

if element.primitiveType == .point

statement, with everything else remaining the same.

This change has been extensively tested and has been shown to work well.