fixed background notes

if the background is fixed then it needs to start tiling
at the corner of the canvas and it needs to tile. the tiling will
start at the first y coord where y-img.height < box.y that is also
a multiple of 0 (the top of the canvas) + the image heights.

 box.y % img.height
 
 box.y = 17
 box.height = 45
 img.height = 10
 
should be at 10
then at 20


box.y + box.y%img.height

img.height * n > box.y <
img.height * n = box.y
n = round up(box.y / img.height)
n = round up(17/10)
n = round up(1.7)
n = 2
draw.y = img.height*2;
stop.y = round up(box.y + box.height / img.height) 
stop.n = round up(6.2)
stop.y = img.height * 7
stop.y = 70

loop from 20 to 70 step 10