compute ideal dictionary size for maximum compression (xz)

[krunch3r76]

xz filter chain includes a dict size which normally cuts off at 64MiB for highest compression (9e) but can be adjusted up to 1.5GiB! heuristically, this can be chosen so that it measures the uncompressed file size.

[krunch3r76]

increasing the dict size imposes exponential increases in memory requirements where the file is decompressed while questionably improving the compression ratio. using the standard 64MiB corresponding to the most extreme default is most feasible. closing.