Change the default compression level for PNG to 5.

The available range is 0..9. Recommendation from
'png_set_compression_level - Linux Standard Base' say 3..6 perform as
well as 9 and do considerably fewer calculations.

Change-Id: Ib2a8e651bd570be13a0d847f61dca6f4b41b74ee
Merge topic '14426-huge-png' into master

21df2adc Change the default compression level for PNG to 5.