lib/deflate_compress: select min_size_to_compress based on level

[ebiggers]

The cutoff for outputting uncompressed data is currently < 16 bytes for
all compression levels. That isn't ideal, since the higher the
compression level, the more we should bother with very small inputs; and
the lower the compression level, the less we should bother.

Use a formula that produces the following cutoffs:

    Level  Cutoff
    -----  ------
    0      56
    1      52
    2      48
    3      44
    4      40
    5      36
    6      32
    7      28
    8      24
    9      20
    10     16
    11     12
    12     8

Update #67