What's new in Qwt 6.2

# Qt >= 4.8

Support for Qt < Qt 4.8 has been dropped, support for Qt6 has been added.

# Includes

Include files, that match the class names are available now. So it is possible to write "#include <QwtPlot>" now instead of "include qwt_plot.h"

# QwtPolar has become part of Qwt

The QwtPolar project ( https://qwtpolar.sourceforge.io ) has been integrated into Qwt.

# MathML Text Renderer has become its own project

The code can be found at https://github.com/uwerat/qwt-mml-dev now and is intended to become a standalone lib.

# Spline

The broken implementation of QwtSpline has been replaced by a bunch of classes offering all sort of functionalities around splines.

The most popular spline approximation/interpolation algos have been implemented:

    - Basis
    - Cardinal
    - ParabolicBlending
    - Akima
    - The one used in a proprietory office package
    - Cubic

# Better OpenGL support

QwtPlotOpenGLCanvas added to support rendering to a QOpenGLWidget

# New plot items

    - QwtPlotVectorField
      A new type of plot item for vector fields

    - QwtPlotGraphicItem
      An item displaying a QwtGraphic image ( f.e used by QwtPlotSvgItem )

# QwtPlotCurve

    - QwtPlotCurve::FilterPointsAggressive
      A fast weeding algo for huge datasets with increasing x or y values

    - QwtPlotCurve::closestPoint
      Is virtual now

    - Line Clipping
      Includes the painter clip now

    - QwtValuePointData
      A new type of data added, where the x values are the index

    - QwtPlotCurve::setSamples
      more type of setters

# QwtPlotSpectrogram

    - QwtPlotSpectrogram::setColorTableSize
      Using individual RGB tables

    - QwtRasterData::setInterval/interval
      API cleanup

    - QwtHueColorMap, QwtSaturationValueColorMap
      New type of color maps

    - QwtMatrixRasterData::BicubicInterpolation
      Implementation of bicubic interpolation added

    - Handling of NaN values
      NaN values are interpreted as gaps, when QwtRasterData::WithoutGaps
      is not set

# Other changes

    - QwtPlotRenderer
      Using QPdfWriter instead of QPrinter, where possible

    - Handling of Unknown Paint Engines
      Not aligning unknown paint engines ( f.e EMF ), QwtNullPaintDevice is not using
      QPaintEngine::User anymore

    - QwtTransform::LogMin/LogMax
      LOG_MIN/LOG_MAX have been reomed ( values differ !

    - qwt_compat.h
      Removed

    - qwtFuzzyGreaterOrEqual/qwtFuzzyLessOrEqual
      Removed

    - qwtGetMin/qwtGetMax
      Removed