Never output jbig2

[hfiguiere]

djpdf should never use jbig2 to compress. While JBIG2 compression is good, it can produce erroneous document by substituting one shape with another:

See https://en.wikipedia.org/wiki/JBIG2#Disadvantages and the associated references.

[Unrud]

A warning message for JBIG2 is already displayed. This should only be a problem with very low DPI values compared to the character size.
To further mitigate the problem, I raised the default threshold for JBIG2 from 85% to 90% and added customizable defaults.

[hfiguiere]

ever saw a warning (at least in the UI)

[hfiguiere]

with the default settings:

before:

after: