write PNG files with compression 9

the quality setting only makes sense for jpegs. Pngs are lossless. Here we only need a setting for compresion level 0-9. But I always would use the max compression.

see the linked post: https://processwire.com/talk/topic/6322-png-image-filesize-after-upload-issue/#entry62030 and the first post of this thread. Also the testcase without that change shows how drastically the filesizes actually are: http://images.pw.nogajski.de/image-sizer-core/formats/quality-vs-compression/