Implement point-moving mutations

Like Alsing does.

This is NOT good enough to replace the normal mode yet, and since people
keep compiling from dev, it's disabled by MIN_POLYS being huge by
default until it works. For your own safety ! Change MIN_POLYS to a
reasonnable value to enable the new mutations (slow).
Improve mutations a little

The new mutations are now enabled. They start after the 50th poly is
added by default.