Avoid "primitive restart" issue

Once https://github.com/KhronosGroup/glTF/pull/1476 gets merged, this line:

https://github.com/KhronosGroup/COLLADA2GLTF/blob/fbbcb4dc1c26d5265393af53add72483f9009efd/src/COLLADA2GLTFWriter.cpp#L730

```
if (index < 65536) {
```

Should be changed to `< 65535` to avoid triggering a primitive restart in certain APIs.

Further details [on this thread](https://github.com/KhronosGroup/glTF/issues/1142#issuecomment-426355904).

After KhronosGroup/glTF#1476 is merged, inspect src/COLLADA2GLTFWriter.cpp at line 730 and review the related discussion in glTF#1142. Update the index boundary to prevent primitive restart in affected APIs; done means the boundary matches the glTF guidance.

Written by the indexing model from the issue text.