#369 polygon clipping affects spline interpolation

[Uwe Rathmann]

For performance reasons polygon clipping is done before curve fitting in QwtPlotCurve::drawLines. This affects the results of the spline interpolation. For local splines ( cubic spline is the only non local one ) the effect can be reduced by adding some dummy points when entering/leaving the canvas. As all implemented spline types have a locality <= 2 it should be good enough to add 2 points each.

Those dummy points have to extend the entering/leaving line to avoid that the invisible connections ( horizontal/vertical lines ) between the leaving/entering points will go into the calculations for the visible lines.