Started Triangle Merging

ChunkFace
- will find all of the merges using the Left->Right; Bottom->Top
algorithm.
- creates "Selections" which will produce the triangles and colors for
the mesh.

ChunkProvider gained the method isRebuildAllowed() for limiting the
number of chunk rebuilds per frame.

Chunk will check with it's ChunkProvider to see if it's allowed to
rebuild the mesh before doing so.