The Exact modifier code had been written to avoid using BMesh but
in the initial release the modifier still converted all Meshes to
BMeshes, and then after running the boolean code on the BMeshes,
converted the result back to a Mesh.
This change skips that. Most of the work here is in getting the
Custom Data layers right. The approach taken is to merge default
layers from all operand meshes into the final result, and then
use the original verts, edges, polys, and loops to copy or interpolate
the appropriate custom data layers from all operands into the result.
