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/ras.cpp

@@ -8,7 +8,6 @@
 */
 
 #include "ras_p.h"
-
 #include "util_p.h"
 
 #include <QDataStream>
@@ -152,8 +151,7 @@ static bool LoadRAS(QDataStream &s, const RasHeader &ras, QImage &img)
     }
 
     // Allocate image
-    img = QImage(ras.Width, ras.Height, QImage::Format_ARGB32);
-
+    img = imageAlloc(ras.Width, ras.Height, QImage::Format_ARGB32);
     if (img.isNull()) {
         return false;
     }