Fix saving files with extreme aspect ratio

Saving failed because thumbnail generation didn't take into account
that scaling a QSize with KeepAspectRatio may round one dimension down
to zero.
This caused even the recovery path to generate an invalid thumbnail
image and saving aborted.

BUG:460624

libs/ui/KisDocument.cpp

@@ -1662,8 +1662,8 @@ QPixmap KisDocument::generatePreview(const QSize& size)
     if (image) {
         QRect bounds = image->bounds();
         QSize originalSize = bounds.size();
-        QSize newSize = bounds.size();
-        newSize.scale(size, Qt::KeepAspectRatio);
+        // QSize may round down one dimension to zero on extreme aspect rations, so ensure 1px minimum
+        QSize newSize = originalSize.scaled(size, Qt::KeepAspectRatio).expandedTo({1, 1});
 
         bool pixelArt = false;
         // determine if the image is pixel art or not

plugins/impex/libkra/kra_converter.cpp

@@ -288,9 +288,10 @@ KisImportExportErrorCode KraConverter::savePreview(KoStore *store)
 {
     QPixmap pix = m_doc->generatePreview(QSize(256, 256));
     QImage preview(pix.toImage().convertToFormat(QImage::Format_ARGB32, Qt::ColorOnly));
-    if (preview.size() == QSize(0,0)) {
+    if (preview.size().isEmpty()) {
         QSize newSize = m_doc->savingImage()->bounds().size();
-        newSize.scale(QSize(256, 256), Qt::KeepAspectRatio);
+        // make sure dimensions are at least one pixel, because extreme aspect ratios may cause rounding to zero
+        newSize = newSize.scaled(QSize(256, 256), Qt::KeepAspectRatio).expandedTo({1, 1});
         preview = QImage(newSize, QImage::Format_ARGB32);
         preview.fill(QColor(0, 0, 0, 0));
     }