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/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;
             }