vcacheoptimizer: Refactor adjacency building
vcacheoptimizer: Refactor adjacency building

This change renames the adjacency structure/functions to triangle
adjacency to avoid conflicts with edge adjacency structure/functions
we'll need for the simplifier.

Also rename triangle_counts to counts and use memset instead of a for
loop to fill the initial counts.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer