Spatial data structure to optimize triangle search in case of unavailable adjacent triangles

[JarkkoPFC]

Meshlets are created by greedily adding the next "best" triangle to the meshlet that's connected to the current meshlet. However, if there are no adjacent triangles available for a meshlet that's being built, we search for an unassigned triangle from the entire mesh that's spatially closest to the meshlet and continue growing the meshlet from there. However, the search is done by linearly iterating through the triangles, which can get quite expensive for larger meshes and significantly slows down the meshlet creation. This task is about adding a spatial data structure to speed up the search.