db8af2f changed rebase() to round down when rebasing times, but

db8af2f changed rebase() to round down when rebasing times, but
did not say why.  This rounding down produces some unexpected results,
e.g. taking 41 at a TCR of 1000 to a TCR of 24 gives 0, which upsets
those who use DCP-o-matic and put in a SRT subtitle ending at ,041 ---
the sub time is rounded down to 0 for a SMPTE DCP.

This commit restores round-to-nearest behaviour.