Trade: fix moving between compressed and uncompressed ImageData.

I mean, yeah, it's all bad, but at least it works now. The upcoming
internal representation will not be this silly CompressedPixelStorage
anymore and then it will become a bit less bad.

For a proper language-lawyer-safe implementation I'd explicitly call
destructors and then in-place-new the other instance and such, but
that's two more branches and thus twice as many chances to mess up.

src/Magnum/Trade/ImageData.cpp

@@ -119,11 +119,15 @@ template<UnsignedInt dimensions> ImageData<dimensions>& ImageData<dimensions>::o
     swap(_dataFlags, other._dataFlags);
     swap(_compressed, other._compressed);
     swap(_flags, other._flags);
-    if(_compressed) {
+    /* Because the CompressedPixelStorage is larger than the
+       uncompressed, copy it if either of the sides is compressed to ensure no
+       compressed properties are lost. The _storage / _compressedStorage and
+       _format / _compressedFormat are unions of trivially copyable contents so
+       copying a type that's not there should be fine. */
+    if(_compressed || other._compressed) {
         swap(_compressedStorage, other._compressedStorage);
         swap(_compressedFormat, other._compressedFormat);
-    }
+    } else {
-    else {
         swap(_storage, other._storage);
         swap(_format, other._format);
     }