Implement a filled surface graph type

Implement a surface graph that fills the area under (or over, depending on the settings?) the graph based on the colorStyle ([GraphsTheme QML Type | Qt Graphs 6.8.1|https://doc.qt.io/qt-6/qml-qtgraphs-graphstheme.html#colorStyle-prop]) property of the theme.

Consider if this could be done in the shader, and add another shading mode for it ([Surface3DSeries QML Type | Qt Graphs 6.8.1)|https://doc.qt.io/qt-6/qml-qtgraphs-surface3dseries.html#shading-prop]