Experimental preview: Vertex sharing for fixed topology files

added import options, off by default: "Merge Vertices (experimental)"

when set to true, will share vertices between polygons when possible.
Note that, this an investigation into a potential optimization.
Seeing reductions, in some cases of 3-4x the number of vertices sent to the GPU.
But, merge process is single threaded and injects per un-merged vertex 2 hash table lookup's (overhead).

Only works in MS. Windows!