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

src/imageformats/dds.json

@@ -0,0 +1,4 @@
+{
+    "Keys": [ "dds" ],
+    "MimeTypes": [ "image/x-dds" ]
+}