Move the dark canvas into PaletteGraphicsView and test it directly

The inverted painting, the rubber band replay and the changed()
receiver lived in DiagramView, which the unit tests cannot link, so the
update-flag regression was only covered through a stand-in view. They
now live in PaletteGraphicsView, a QGraphicsView subclass with no
other dependency, and DiagramView derives from it. The view tells a
subclass through paintingInverted(bool) when it renders for an
inverted display; DiagramView forwards that to the diagram. The grid
dot rule moves out of Diagram::drawBackground into
QET::Palette::gridDotColor().

tst_qetpalette now links the real class: gridDotColorSoftensInvertedDots,
paletteViewFollowsThePalette (light sheet, dark sheet at text contrast
with a red box still red and the paintingInverted calls in order, back
to light), paletteViewKeepsSceneUpdatesFlowing (three whole-scene
updates and a selection each repaint, scene set after construction),
paletteViewDrawsTheRubberBand.

sources/palettegraphicsview.h

@@ -0,0 +1,70 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef PALETTE_GRAPHICS_VIEW_H
+#define PALETTE_GRAPHICS_VIEW_H
+
+#include <QGraphicsView>
+
+class QPainter;
+
+/**
+	A QGraphicsView that shows its scene with inverted lightness while its
+	palette is dark: white becomes the palette's Base color, black its Text
+	color, and colored strokes keep their hue. The scene itself is left as
+	drawn, so printing and exporting it still give black on white. On a
+	light palette the view paints exactly as QGraphicsView does.
+
+	The view paints through QGraphicsView::render() into an image and blits
+	the inverted image. QGraphicsView delivers scene updates straight to
+	its viewport when nobody listens to QGraphicsScene::changed(), and in
+	that mode the scene clears its "update everything" flag only when the
+	items are painted straight onto the viewport, which never happens
+	here: from the second QGraphicsScene::update() on, every scene update
+	and every item update would wait for an unrelated repaint. So the view
+	listens to changed() on every scene it is given, which makes the scene
+	clear the flag before it emits. Set the scene through this class, not
+	through a QGraphicsView pointer.
+*/
+class PaletteGraphicsView : public QGraphicsView
+{
+	Q_OBJECT
+
+	public:
+		explicit PaletteGraphicsView(QWidget *parent = nullptr);
+		explicit PaletteGraphicsView(QGraphicsScene *scene, QWidget *parent = nullptr);
+
+		void setScene(QGraphicsScene *scene);
+		bool invertsLightness() const;
+
+	protected:
+		void paintEvent(QPaintEvent *event) override;
+		/**
+			Called with true right before the scene is rendered for an
+			inverted display and with false right after, so a scene can
+			adapt what it draws (a softer grid, for instance). Does
+			nothing by default.
+		*/
+		virtual void paintingInverted(bool inverted);
+
+	private:
+		void listenToScene(QGraphicsScene *scene);
+		void paintInverted(const QRect &area);
+		void drawRubberBand(QPainter &painter);
+};
+
+#endif