Add utility for simplifying flat mesh surfaces

[gkjohnson]

During the process of applying CSG to a mesh flat surfaces become unnecessarily complex. During lulls in user operation it would be good to remesh brushes so they use a minimal number of triangles while still being connected - ie retriangulate flat / coplanar surfaces.

• Find all large, flat surfaces while retaining half edges
• Mark shared vertices as being removeable or not - ie required to define the contour of a shape.
• Remove unnecessary vertices from the new shapes
• Earcut to triangulate the surfaces

Alternative:

• Find all polygon contours
• Remove vertices along straight edges
• Find connected edges of contours and join them (check for coplanarity)
• Ensure holes are retained
• Use earcut to produce a new geometry

TODO:

• How do deal with uv or other buffer attributes?

[johncalvinyoung]

For a variety of use cases, it'd be good to make this a purely optional part of the workflow--I have applications where retaining the position & connectivity of any un-changed vertices is mildly important

[gkjohnson]

Possibly related:

mrdoob/three.js#10517