Models with slightly more than or equal to 65535 vertices are bumped to considerably higher vertices in importer

[AtlasLinden]

Environment

Second Life Project Gltf Mesh Import 7.1.14.15976006598

Description

The below attached model (which is a simple flat plane) has 65535 vertices. When this is selected for upload into the viewer, the importer recalculates this to have a whopping 131075 vertices.

As mentioned in #4242

Reproduction steps

Select to upload the following GLB model: 65535vertices.zip

[akleshchev]

Todo: osrt vertices (or triangles?) by position before spliting, that should insure they are grouped together when spliting.

[maxim-productengine]

Here's a comparison using test models from Atlas:
65536vertices.glb
blender: 65536
Viewer before: 131075
Viewer after: 65551

65535vertices.glb
blender: 65536
Viewer before: 131075
Viewer after: 65548

There is an improvement for other models I tested too, but not so huge.