Better folder layout for better scaling

[tokee]

With the default settings, creating static tiles for 1 million images will result in 12 million tile files in a single folder. With many file systems this is detrimental to performance. This can be solved by creating a custom tileSource for OpenSeadragon with more sub-folder, such as level/x/x_y.jpg instead of level/x_y.jpg.

This does not change the basic problem: A large amount of small files. Using an image server with pyramid TIFFs solves this, but this introduces a server-side requirement and puts us back to square 1: Having to produce 1 very large image.

A Frankenstein solution would be to create multiple pyramid-TIFFs, each one containing a subset of the tiles. This would make it possible to do near-infinite scale collages, at the cost of a significant increase in both implementation and infrastructure complexity.

[tokee]

The implemented solution is the fairly-large-scale one: If enabled, tiles are bucked together in sub-folders. The solution only uses a single sub-level, so the performance of some file systems will gradually deteriorate when creating (and possibly also when viewing) collages of more than 30 million images (using the default setup).

Guesstimating a bit, things will probably be okay up to a few hundred million images. Should anyone need to go beyond that, I'll be happy to write the code for using multi-level sub-foldering for the tiles.