Heatmap fix segment repeating

[NodudeWasTaken]

This commit fixes that segments repeat when they shouldnt
Here's a heatmap

Notice that in a section that is empty, there is action for some reason.
Here's the new version:

No action where there is no action

The issue for nerds:

for i := 0; i < numSegments; i++ {
	segmentTS := int64(float64(i) / float64(numSegments))

	// Empty segment - fill it with the previous up to backfillThreshold ms
	if segments[i].count == 0 {
		if segmentTS-lastSegment.at < backfillThreshold {

In this code, segmentTS is always zero.
Why?
Well, if numSegments is 50, and i is 25, this is 25/50, which is 0.5.
This is a calculation that approaches 1 as you reach numSegments, and its an integer, so its zero until i=numSegments (cant happen).
Here's the new code:

for i := 0; i < numSegments; i++ {
	segmentTS := (maxts / int64(numSegments)) * int64(i)

In this, we take the duration, divided by the number of segments, and multiplied by the current segment.
This should give us the approximate time of this segment.
You cant use ".at", since this segment is empty and ".at" was never set.

I also cleanup the speed code to use interactive speed instead of slope calculation, as slope seemed redundant and unnecessarily complicated.