Support injecting multiple MATIDs per mesh
Future BASEMENT capabilities may use additional material IDs beyond indices 0 (element material) and 1 (element face elevation).
Triangle does not support triangulation with multiple element attributes, so the modified breaklines would need to be triangulated separately. These extra triangulations would not require any mesh quality constraints and would be more compatible to elevation meshes with MATIDs.
The materials generated for these auxiliary meshes could then be injected into the main quality mesh as extra mesh indices. QGIS 3.16 already supports these additional materials and will name them "Auxiliary Material #" in the GUI. The injection of these materials is comparable in effort to mesh interpolation, although the copying of materials is considerably cheaper than the barycentric interpolation. Since the number and position of elements may change from one auxiliary mesh to the other, this step needs to be repeated for each.
It would be possible to hide all of these steps behind an advanced quality meshing step, where the user specifies two or more pairs of breaklines and region markers, with quality meshing, auxiliary mesh generation, and MATID injection performed automatically. However, this would a significant change to the quality meshing workflow that affects many parts of BASEmesh operation - so while possible in general, we will not implement this as part of the quality meshing workflow for the time being.