Slightly improved approx. meshing by allowing to use different offsets when iterating over rows and columns

This is a slight improvement of approximate meshing by using different offsets for iterating over rows and columns when constructing the approximate mesh. This may make less sense for RGB-D images, but can be helpful, e.g., when processing 3D scans composed of multiple 2D laser range scans where the point density in the individual 2D scans is considerably higher than between the scans.

Regarding API changes, two functions are added: setTrianglePixelSizeRows and setTrianglePixelColumns for setting the two increments. setTrianglePixelSize is still available to set both equally (as was the behavior beforehand).