Make PNJ the default map image io backend

[jpenilla]

Configs with (effectively) default image io settings will be switched to use PNJ. Otherwise, they will continue to use BufferedImage. Acceptable values for the backend option are pnj and bufferedimage.

Some basic testing shows that PNJ gives compression near the max setting for BufferedImage while taking less time.

The following results are for rendering the same map twice in a row (after a warmup run) averaged over a couple of runs (time), and for the whole tiles directory for the dimension (size). Time is what was spent in MapImage#save on the image io thread as measured by spark (async profiler).

Description	Size	Time
PNJ	724K	1400ms
BufferedImage compress-images=enabled, value=1	712K	2300ms
BufferedImage compress-images=disabled	804K	950ms

Questions to answer before merging:

• Do we want to change the default or just add an option?
  • Typically when users are bottlenecked by image io it's due to disk speed, not encoding/decoding.
  • We may want to run more benchmarks separating encoding/decoding. The above table profiles the entire MapImage#save method (which includes encode and decode).
• Do we want to change the config layout to better allow for support of other image formats in the future (i.e. lossless/lossy webp)? Or should we leave that for when other image formats are implemented?