Re-added DDS plugin support

Fork of [Qt 5.6 DDS plugin](https://code.qt.io/cgit/qt/qtimageformats.git/tree/src/plugins/imageformats/dds/qddshandler.cpp?h=5.6) under LGPL2.1.

- Merged all files in dds_p.h and dds.cpp
- Added support for Qt 6 image allocation limit
- Added checks for null image and datastream errors
- The plugin is disabled by default

CCBUG: 380956

Closes: #12

autotests/CMakeLists.txt

@@ -76,6 +76,11 @@ kimageformats_read_tests(
     tga
 )
 
+if(KIMAGEFORMATS_DDS)
+    kimageformats_read_tests(dds)
+    kimageformats_write_tests(dds)
+endif()
+
 if (KF6Archive_FOUND)
     kimageformats_read_tests(
         kra