Changing block layout for alignment

I changed the way the XY block layout is chosen. Starting from the regular grid:

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|    |    |
|----|----|
|    |    |
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The old code placed a block of the same size in the middle of this grid. All tiles that touched a block were pulled from the DB and aligned. Only tiles that had their midpoint completely within a block were kept for global alignment. This lead to 1) no overlap at the boundaries of the grid; 2) a relatively small layer of tiles that were not kept after alignment (which is bad because the boundary tiles could be aligned badly).
The new code simply grows the blocks by a factor of 2 in every dimension, which guarantees sufficient overlap. After block-alignment and before global interpolation, the outermost 25% in every dimension is shaved off.

saalfeldlab / render