Fix Undercuts

Within the MR::FixUndercuts namespace, functions are provided to eliminate "undercut" regions. The primary function is:

void MR::FixUndercuts::fixUndercuts(Mesh &mesh, const Vector3f &upDirection,
                                     float voxelSize=0.0f, float bottomExtension=0.0f);

An overloaded version includes a selectedArea parameter that restricts the operation to a specific part of the mesh. This function accepts a mesh, an "upward direction" vector (typically opposite to the direction of gravity), along with the voxel resolution voxelSize and a bottom extension thickness bottomExtension.

The implementation approach is analogous to anisotropic offsetting: first, the mesh is voxelized along the specified direction with the resolution defined by voxelSize, and then voids are "filled" upward, ensuring that all faces receive support relative to the upDirection. Following the invocation of fixUndercuts, the previously undercut or overhanging regions will be filled with voxels and reconstructed into a solid mesh. Prior to calling, one can utilize MR::FixUndercuts::findUndercuts(mesh, upDir, outFaces) to identify the undercut faces (or vertices) and assess the undercut area using functions like getUndercutAreaMetric. Overall, fixUndercuts effectively expands or offsets the mesh in the specified direction, thereby eliminating excessive undercut angles.