Cancel `AudioParam::set_target_at_time` when close enough to target value

[b-ma]

trafficstars

from #20

set_target_at_time can run forever as it as no "real" end time or value, a possible performance improvement would be to internally cancel the event when intrisic_value is close enough to the target (note that Chrome implements such strategy)

[b-ma]

// at the end of the block, avoid doing this for each sample
let diff = target - intrisic_value;
if diff.is_subnormal() {
    intrisic_value = target;
    // insert `set_value` event in timeline at next block
}

could do the trick

[b-ma]

Fixed in #190