Fix transparency in 16 bit and 24 bit ico files

As a result of the fix for QTBUG-75214, Qt inadvertently no longer
reads the AND mask that specifies transparency for 16-bit and 24-bit
ico files. This is because it tries to detect 32-bit icons by checking
icoAttrib.depth == 32, but icoAttrib.depth is set to the depth of the
QImage, not the depth of the icon, and 32-bit QImage is used for all of
the non-indexed cases (16-bit, 24-bit and 32-bit.)

This commit instead uses icoAttrib.nbits, which should reliably
determine whether or not the icon is 32-bit. This makes the behavior
consistent with other ico reading software, including Windows.

Also, adds a unit test that verifies correct behavior of icon masks,
checking for both QTBUG-75214 and QTBUG-113319.

Amends 1d128ed1df.

Fixes: QTBUG-113319
Change-Id: I89ac86ff16054c8925fff6afc8c530fa737f8385
Pick-to: 6.6 6.5 6.2 5.15
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/plugins/imageformats/ico/qicohandler.cpp

@@ -499,7 +499,7 @@ QImage ICOReader::iconAt(int index)
                     if (!image.isNull()) {
                         readBMP(image);
                         if (!image.isNull()) {
-                            if (icoAttrib.depth == 32) {
+                            if (icoAttrib.nbits == 32) {
                                 img = std::move(image).convertToFormat(QImage::Format_ARGB32_Premultiplied);
                             } else {
                                 QImage mask(image.width(), image.height(), QImage::Format_Mono);

tests/auto/gui/image/qicoimageformat/tst_qicoimageformat.cpp

@@ -27,6 +27,8 @@ private slots:
     void pngCompression();
     void write_data();
     void write();
+    void icoMask_data();
+    void icoMask();
 
 private:
     QString m_IconPath;
@@ -319,6 +321,33 @@ void tst_QIcoImageFormat::write()
     }
 }
 
+void tst_QIcoImageFormat::icoMask_data()
+{
+    QTest::addColumn<QString>("inFile");
+    QTest::addColumn<QString>("outFile");
+
+    QTest::newRow("24bpp") << "masked/24bpp.ico" << "masked/24bpp.png";
+    QTest::newRow("32bpp") << "masked/32bpp.ico" << "masked/32bpp.png";
+}
+
+void tst_QIcoImageFormat::icoMask()
+{
+    QFETCH(QString, inFile);
+    QFETCH(QString, outFile);
+
+    QImage inImage;
+    QImageReader inReader(m_IconPath + QLatin1Char('/') + inFile);
+    inReader.read(&inImage);
+
+    QImage outImage;
+    QImageReader outReader(m_IconPath + QLatin1Char('/') + outFile);
+    outReader.read(&outImage);
+    outImage.setColorSpace(inImage.colorSpace());
+    outImage = outImage.convertToFormat(inImage.format());
+
+    QCOMPARE(inImage, outImage);
+}
+
 QTEST_MAIN(tst_QIcoImageFormat)
 #include "tst_qicoimageformat.moc"