Add mesh decimation filter (surface or volume)

[imikejackson]

[imikejackson]

Current codes need to be merged into a plugin.

[wlenthe]

The biggest hurdle in getting this ported to the current branch is that the algorithm requires a dynamic mesh data structure to be tractable. This is a bit painful to write, particularly given that the triple lines are non-manifold so a simple half-edge structure isn't suitable. If you can pick an existing mesh library to use (or someone else feels like writing something from scratch) the actual decimation should be easy to implement.