Attempt at line-per-line loading of PNGs.
Lets me 'load' pippin's huge 31129 x 28957 file (with commented out layer preview and bird's eye). Loading itself takes something like 5 minutes, but doesn't take any memory with good swap settings. Afterwards, Krita consumes memory like a madman, I'm guessing part of it is the display code (changing zoom level takes about 2 minutes,