Rows are now output from the source tiles and the mask image which controls brightness of
individual tiles in each row. The rows can then be combined using montage or PIL further.

The choice was made to use this so that the size of each generated image could be easily
controlled to manage memory usage (dev'd on a 7 year old laptop...)

Tiles are 'scored' by a separate function to control their display based on the mask pixel
for that particular tile in the ultimate image.