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.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);