Simplify mesh until target count is reached
Simplify mesh until target count is reached

We keep the vertex quadrics around and accumulate q0 error into q1
during v0->v1 collapse to preserve the shape better.

To make it simple, instead of a priority queue and error update we just
split simplification into multiple passes; within each pass we forbid
collapses that share the source/target vertex, both to simplify the
algorithm and to spread the collapses around the mesh.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer