Fix image allocation with Qt 6

To make the plugins fail to allocate if the image size is greater than QImageReader::allocationLimit() it is necessary to allocate the image with QImageIOHandler::allocateImage().

Note that not all plugins have been changed and some others are not tested in the CI (maybe due to missing libraries).

PS: the following message is printed by QImageIOHandler::allocateImage() if the size is exceeded: "qt.gui.imageio: QImageIOHandler: Rejecting image as it exceeds the current allocation limit of XXX megabytes"

src/imageformats/util_p.h

@@ -1,10 +1,42 @@
 /*
     SPDX-FileCopyrightText: 2022 Albert Astals Cid <aacid@kde.org>
+    SPDX-FileCopyrightText: 2022 Mirco Miranda <mircomir@outlook.com>
 
     SPDX-License-Identifier: LGPL-2.0-or-later
 */
 
+#ifndef UTIL_P_H
+#define UTIL_P_H
+
 #include <limits>
 
+#include <QImage>
+
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+#include <QImageIOHandler>
+#endif
+
 // QVector uses some extra space for stuff, hence the 32 here suggested by Thiago Macieira
 static constexpr int kMaxQVectorSize = std::numeric_limits<int>::max() - 32;
+
+// On Qt 6 to make the plugins fail to allocate if the image size is greater than QImageReader::allocationLimit()
+// it is necessary to allocate the image with QImageIOHandler::allocateImage().
+inline QImage imageAlloc(const QSize &size, const QImage::Format &format)
+{
+    QImage img;
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
+    img = QImage(size, format);
+#else
+    if (!QImageIOHandler::allocateImage(size, format, &img)) {
+        img = QImage(); // paranoia
+    }
+#endif
+    return img;
+}
+
+inline QImage imageAlloc(qint32 width, qint32 height, const QImage::Format &format)
+{
+    return imageAlloc(QSize(width, height), format);
+}
+
+#endif // UTIL_P_H