Change default compression quality from 90 to 89

Qt's PNG ImageWriter (ab)uses setQuality() for setCompression(),
which causes "no compression" for any quality value larger than 89.
See https://bugreports.qt.io/browse/QTBUG-43618

For screenshots, which usually have large areas of flat colors,
uncompressed files are very large (e.g. 24 MB for a 4K screen).
Sharing screenshots directly is quite common, so users won't notice
immediately that they are sharing big files.

Changing the quality value from 90 to 89 doesn't severely degrade
quality for JPEG files until we find a better solution,

CCBUG: 406263