Compute a picture's displayed pixmap from its source

The displayed pixmap and imageSource (original, crop rectangle,
transparent colours) were two undo values that had to change together,
although the pixmap follows from the source. An action that updated
one and not the other would bring back the bug fixed in #1310.

setImageSource() now recomputes the displayed pixmap, and crop, colour
key, mirror and replace push one undo step on imageSource alone. A
plain QUndoCommand holds the property change, so two identical actions
in a row stay two steps. Loading a project still shows the saved pixmap
as before.

tst_imagecropundo also checks the size of the saved picture after undo
and after redo.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/qttest/tst_imagecropundo.cpp

@@ -25,6 +25,7 @@
 #include <QProcess>
 #include <QProcessEnvironment>
 #include <QRegularExpression>
+#include <QSize>
 #include <QTemporaryDir>
 
 /**
@@ -77,6 +78,16 @@ class tst_imagecropundo : public QObject
 		QFile f(path);
 		return f.open(QIODevice::ReadOnly) ? f.readAll() : QByteArray();
 	}
+	// The size of the picture as shown, from the <image> element's own
+	// text (its first text node: a cropped picture carries <image_base>
+	// as a child too).
+	static QSize shownSize(const QByteArray &xml)
+	{
+		const QRegularExpression image(QStringLiteral("<image\\b[^>]*>\\s*([A-Za-z0-9+/=]+)"));
+		const QRegularExpressionMatch m = image.match(QString::fromUtf8(xml));
+		if (!m.hasMatch()) return {};
+		return QImage::fromData(QByteArray::fromBase64(m.captured(1).toLatin1())).size();
+	}
 
 private slots:
 	void initTestCase()
@@ -125,6 +136,9 @@ qet.log('PROBE ' + JSON.stringify(r));
 		const QByteArray undone_xml = read(undone);
 		QVERIFY(undone_xml.contains("<image "));
 		QVERIFY2(!undone_xml.contains("<crop "), "an undone crop was saved");
+		// The picture shown follows: whole after undo, the kept region after redo.
+		QCOMPARE(shownSize(undone_xml), QSize(40, 30));
+		QCOMPARE(shownSize(read(redone)), QSize(20, 10));
 		const QRegularExpressionMatch crop =
 				QRegularExpression(QStringLiteral("<crop ([^>]*)/>")).match(QString::fromUtf8(read(redone)));
 		QVERIFY2(crop.hasMatch(), "the redone crop was not saved");