Non convex, non-triangulated mesh is triangulated incorrectly by importer

[MathemanFlo]

ArmorPaint version: ArmorPaint.org - v0.9 (2022-02-12) - 949c513 Windows - Direct3D11

Issue description: ArmorPaint has an inbuilt triangulator for non triangular faces for a while. This feature works unreliably in some cases for non convex, non-triangulated meshes.

Steps to reproduce: I have made some example files non-convex-examples.zip Example of what goes wrong: The red circle shows a crossing of two edges, i.e. a non valid mesh situation.