Fixes of little patches which look like UV artifacts.

[mifth]

No description provided.

[mifth]

This is a fix for little patches which appear on highpoly models. It was originally made in this PR #138 but I've separated it.

I've attached a model where you could test the issue:
Monkey_smart_triangulation.zip

Run this code:

xatlas::Atlas* atlas = xatlas::Create();
xatlas::AddMeshError meshError = xatlas::AddMesh(atlas, meshDecl);  // Add your Mesh
xatlas::ChartOptions chartOptions = xatlas::ChartOptions();
xatlas::PackOptions packOptions = xatlas::PackOptions();
xatlas::Generate(atlas, chartOptions, packOptions);

Before changes:

After changes: