AllPlot - Smoothing also for "Left Border"

Problem:
... data points of "Smoothing" settings are set to Zero in result
... this cause e.g. Zero Altitude Values when Plot Smoothing is active
Solution:
... also most left values are "smoothed" now (using only the datapoint
left of them to create the average)
Merge pull request #1059 from Joern-R/pull2

AllPlot - Smoothing also for "Left Border"