Since the lightmapper can give quite unexpected results when the triangles in the unwrap are overlapping or are outside of the [0;1]x[0;1] range we should validate that, so that the user knows where does the weirdness come from.

We already check if the UVs are outside of the [0;1]x[0;1] range and emit a warning, also Beast skips triangles that are entirely outside of that range (see case 442606).

When importing second UVs we should check for overlapping triangles and if there are any, we should visualize the problem for the user, so that it's easier for him to fix it in his 3D app of choice.