Refactor: Split out saveJpeg/saveGif/savePng functions
Create "empty.jpg" when not saving transparent tiles

That way the Google Maps implementation can use it right away; no more need to manually create the image.
Also allow subfolder-based exports (z/x/y.jpg) instead of purely filebased ones (z_x_y.jpg)
Remove the option "Don't save transparent tiles"

It requires extra logic in the Google Maps implementation and will fail for a lot of people. Better to have a safe default instead of one breaking it for almost everyone.