resample: Avoid off-by-1 errors in PTS calcs.

The rounding used in the PTS calculations in filter_frame() does
not actually match the number of samples output by the resampler.
This leads to off-by-1 errors in the timestamps indicating gaps and
underruns, even when the input timestamps are all contiguous.

Bug-Id: 753

Signed-off-by: Anton Khirnov <anton@khirnov.net>