function takes video lengths as input.
It totals these lengths and divides by $chapters to find chapter
points of the combined videos.
It then cycles through the videos lengths, creating chapter points
appropriately.
Any remaining seconds after the last chapter point in a video is made,
are passed onto the next video.
example:
we will use 5 chapters
video lengths 60 120 180
total: 360
chapter length is therefore: 72 seconds
chapter points: 0 72 144 216 286
video1: 0 (remainder 12)
video2: 60 (remainder 60 )
video3: 12 84 156