DDS: fix buffer overflow in readCubeMap

The issue was identified by OSS Fuzz and the feature was not covered by our tests.

- Added earth-cubemap.dds under MIT licenses taken from [Open Toolkit library](https://github.com/mono/opentk/tree/main/Source/Examples/Data/Textures)
- Fix a wrong image size returned by a cubemap image
- Read test skips .license files

autotests/read/dds/earth-cubemap.license

@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: Copyright (c) 2006 - 2010 The Open Toolkit library.
+SPDX-License-Identifier: MIT

autotests/readtest.cpp

@@ -263,7 +263,7 @@ int main(int argc, char **argv)
         }
         for (const QFileInfo &fi : lstImgDir) {
             TemplateImage timg(fi);
-            if (timg.isTemplate()) {
+            if (timg.isTemplate() || timg.isLicense()) {
                 continue;
             }
 

autotests/templateimage.cpp

@@ -28,6 +28,11 @@ bool TemplateImage::isTemplate() const
     return false;
 }
 
+bool TemplateImage::isLicense() const
+{
+    return !m_fi.suffix().compare(QStringLiteral("license"), Qt::CaseInsensitive);
+}
+
 QFileInfo TemplateImage::compareImage(TestFlags &flags, QString& comment) const
 {
     auto fi = jsonImage(flags, comment);

autotests/templateimage.h

@@ -47,6 +47,12 @@ public:
      */
     bool isTemplate() const;
 
+    /*!
+     * \brief isLicense
+     * \return True if the file suffix is .license
+     */
+    bool isLicense() const;
+
     /*!
      * \brief compareImage
      * \param flags Flags for modifying test behavior (e.g. image format not supported by current Qt version).