Better simplify wrt. mip-maps

[davids91]

Based on implementation

shocovox/src/octree/detail.rs

Line 213 in 9f0b810

pub(crate) fn subdivide_leaf_to_nodes(&mut self, node_key: usize, target_octant: usize) {

• During Leaf node simplification,

  shocovox/src/octree/update.rs

  Line 818 in 4c7d89f

  NodeContent::Leaf(bricks) => {

  do not check if all bricks are equal, instead check if the leaf in itself can be represented by one brick. That essentially means for one voxel in the larger brick, at least 8 voxels need to have the same content ( depending on brick DIM ).
• Add a testcase for this simplification