When using --cycles-resumable-num-chunks N to render a subset of the
samples, having N close to the total number of samples causes rounding
For example, a file configured for 250 samples and 150 chunks should
have 1.6666 sample per chunk. The old code rounded this to 2 samples per
chunk, which would result in too many samples being rendered. When
rendering a single chunk this doesn't matter much, but when larger chunk
ranges are rendered with --cycles-resumable-start-chunk and
--cycles-resumable-end-chunk the rounding errors start to add up.
By multiplying with the number of chunks to render first, and only round
to integers after that, this issue is solved. In the above example,
rendering 3 chunks will correctly render 5 samples rather than 6.