Extend thumb/image -> overlay/watermark method with async/jobs generation

the most easiest way i found (for my knowledge) was to add it to focusCrop, while the overlay method worked that way, it seemed to break other things such as the usual methods (resize, crop) not outputting images anymore, even though no obvious errors can be seen…

i guess i’ll try giving that another shot as my local server is running 7.3.1… @flokosiol