When the (float) bitrate is returned, it is implicitely converted to an
int64 value, merely discarding the fractional part.
However the bitrate of a CBR track can vary a bit due to timestamp
precision loss after clock conversion (this can affect MPEG-TS audio
tracks). So a bitrate like 191999.999... results in 191999 when
being returned - instead of 192000.
To fix this, apply proper rounding, as already done for the "old" case.
Hereby refactoring the "old" case to also use `llrint`.