Introduce invertColor function

This gives meaning to this repeated invocation. And it also
allows us to handle achromatic colors properly, as we would
otherwise risk runtime warnings when encountering those.

kdevplatform/util/widgetcolorizer.cpp

@@ -77,6 +77,16 @@ struct FormatRange {
     QTextCharFormat format;
 };
 
+QColor invertColor(const QColor& color)
+{
+    auto hue = color.hsvHue();
+    if (hue == -1) {
+        // achromatic color
+        hue = 0;
+    }
+    return QColor::fromHsv(hue, color.hsvSaturation(), 255 - color.value());
+}
+
 void collectRanges(QTextFrame* frame, const QColor& fgcolor, const QColor& bgcolor, bool bgSet,
                    std::vector<FormatRange>& ranges)
 {
@@ -110,13 +120,13 @@ void collectRanges(QTextFrame* frame, const QColor& fgcolor, const QColor& bgcol
                         if (bg.valueF() > 0.3) {
                             if (fmt.hasProperty(QTextFormat::BackgroundBrush) && bg.valueF() > 0.5
                                 && bg.hsvSaturationF() < 0.08) {
-                                bg = QColor::fromHsv(bg.hsvHue(), bg.hsvSaturation(), 255 - bg.value());
+                                bg = invertColor(bg);
                                 fmt.setBackground(bg);
                                 if (fg.valueF() < 0.7) {
                                     fmt.setForeground(WidgetColorizer::blendForeground(fg, 1.0, fgcolor, bg));
                                 }
                             } else if (fg.valueF() > 0.5 && fg.hsvSaturationF() < 0.08) {
-                                fg = QColor::fromHsv(fg.hsvHue(), fg.hsvSaturation(), 255 - fg.value());
+                                fg = invertColor(fg);
                                 fmt.setForeground(fg);
                             }
                         }