Anchor windows to the world grid #70

Background

The sampler decides two things: where windows go, and what starting static each one is given. Both are worked out from the region that was asked for, so a window's position and its static depend on which request produced it rather than on where in the world it sits.

That has two consequences. The same piece of world comes out differently depending on how it was asked for, because a different set of windows covers it each time. And two tiles side by side start from unrelated static, so terrain does not carry across the join between them.

For example, asking for a region at 0,0 and then for one at 2,2 that overlaps it. The overlapping area is covered by different windows in each request, drawing different static, so the two answers disagree about terrain that is meant to be the same place.

Description

Move both decisions onto the world. A window sits where the world grid puts it, and draws its static from the seed together with that position.

Once both are true a window is the same window in every request that touches it. Everything else in this epic rests on that. A tile cannot record which windows it has if a window does not mean the same thing twice.

Do #61 before #58. The static is worked out from a window's position, so the position has to be fixed to the world before it means anything.