Log Message:
qwt6-qt5: updated to 6.3

Qwt 6.3

QwtPainter

QwtPainter::polylineSplitting has been reenabled. Unfortunately the Qt raster \ 
paint engine still has serious performance issues with rendering long polylines.

QwtPlotCurve

- QwtPlotCurve::adjacentPoint
  for monotonic curves: using a binary search algo

- QwtPlotCurve::interpolatedValueAt
  for monotonic curves: using a binary search algo
QwtSeriesData

- wrong inheritances fixed
- firstSample, lastSample added
QwtPlotIntervalCurve

QwtPlotIntervalCurve::TubeAsLines mode added. A performance optimization
for rendering huge datasets.
Bugfixes/Adjustments

Bugfixes/Adjustments - often needed because of new Qt developments