Switch filter code to relative timestamps
Part of #36 (closed)
On Dell XPS devices the touchpad wakeup time gives us wrong
SYN_REPORT timestamps. The first three events are too close together, usually t, t+2ms, t+7ms when the hw itself is more like t, t+7ms, t+120ms. With the last event reflecting a 120ms movement (not 7ms) the delta causes a pointer jump.
The filter code doesn't care about the absolute value of the timestamp, it's only used to calculate deltas based on previous time stamps. So we should be able to pass a delta timestamp for the current event in, instead of the absolute time.
This means we can fix the time on-the-fly and have the accel code do the right thing.