Previous changes set value band end time to be the same as the next
value band's start time. However this caused the problem that there is
no way of telling during rendering where exactly within the previous
value band did the band's value go out of dead band and since t_end does
not tell us this. This means that line-rendering (not stepped sample
rendering) will get drawn invalidly if t_end > t_begin within a value
band.

The fix was to set t_end to the last timestamp where the value was still
within deadband, i.e:
```
|t0    t0_end  |               VALUE BAND #0 t0 < t0_end
|      |       |t1      |      VALUE BAND #1 t1 = t1_end
|      |       |t1_end  |
|      |       |        |t2... VALUE BAND #2
ts0----ts1---- ts2------ts3---
v=0    v=0     v=1      v=2
```

gitlab #35

Change-Id: I073d1aed0d3ce0ffe30081937fdd7ff1c22dc107