centroid on border

[zsiki]

pointOnSurface sometimes returns centroid on the border of the polygon. It happens when the borderlines are horizontal and vertical (e.g. result from raster to vector).
It happens with the previous custom algorithm for QGIS < 2.4, too.
Let's improve that algorithm and add a new checkbox avoid centroids on polygon border.
It was reported by Richard Metcalfe.