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/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).