Trade: make it possible to attach importer state to image data.

The data are immutable by design, so it's a special "move constructor".
Usable when a plugin in proxying image import to external importers
(such as AnyImageImporter) and wants to attach its own state to the
result.

src/Magnum/Trade/ImageData.cpp

@@ -41,6 +41,10 @@ template<UnsignedInt dimensions> ImageData<dimensions>::ImageData(const Compress
 
 template<UnsignedInt dimensions> ImageData<dimensions>::ImageData(const CompressedPixelStorage storage, const UnsignedInt format, const VectorTypeFor<dimensions, Int>& size, Containers::Array<char>&& data, const void* const importerState) noexcept: ImageData{storage, compressedPixelFormatWrap(format), size, std::move(data), importerState} {}
 
+template<UnsignedInt dimensions> ImageData<dimensions>::ImageData(ImageData<dimensions>&& other, const void* const importerState) noexcept: ImageData{std::move(other)} {
+    _importerState = importerState;
+}
+
 template<UnsignedInt dimensions> PixelStorage ImageData<dimensions>::storage() const {
     CORRADE_ASSERT(!_compressed, "Trade::ImageData::storage(): the image is compressed", {});
     return _storage;

src/Magnum/Trade/ImageData.h

@@ -218,6 +218,17 @@ template<UnsignedInt dimensions> class ImageData {
          */
         template<class T> explicit ImageData(CompressedPixelStorage storage, T format, const VectorTypeFor<dimensions, Int>& size, Containers::Array<char>&& data, const void* importerState = nullptr) noexcept;
 
+        /**
+         * @brief Construct from existing data with attached importer state
+         *
+         * Useful in cases where importer plugins proxy image loading through
+         * other importers but want to attach its own importer state to the
+         * imported data. Importer state from the @p other object is replaced
+         * with @p importerState, data ownership is transferred and everything
+         * else stays the same.
+         */
+        explicit ImageData(ImageData<dimensions>&& other, const void* importerState) noexcept;
+
         /** @brief Copying is not allowed */
         ImageData(const ImageData<dimensions>&) = delete;
 

src/Magnum/Trade/Test/ImageDataTest.cpp

@@ -49,6 +49,9 @@ struct ImageDataTest: TestSuite::Tester {
     void constructMoveCompressedGeneric();
     void constructMoveCompressedImplementationSpecific();
 
+    void constructMoveAttachState();
+    void constructMoveCompressedAttachState();
+
     void toViewGeneric();
     void toViewImplementationSpecific();
     void toViewCompressedGeneric();
@@ -77,6 +80,9 @@ ImageDataTest::ImageDataTest() {
               &ImageDataTest::constructMoveCompressedGeneric,
               &ImageDataTest::constructMoveCompressedImplementationSpecific,
 
+              &ImageDataTest::constructMoveAttachState,
+              &ImageDataTest::constructMoveCompressedAttachState,
+
               &ImageDataTest::toViewGeneric,
               &ImageDataTest::toViewImplementationSpecific,
               &ImageDataTest::toViewCompressedGeneric,
@@ -440,6 +446,47 @@ void ImageDataTest::constructMoveCompressedImplementationSpecific() {
     CORRADE_COMPARE(c.importerState(), &state);
 }
 
+void ImageDataTest::constructMoveAttachState() {
+    auto data = new char[3*6];
+    int stateOld, stateNew;
+    ImageData2D a{PixelStorage{}.setAlignment(1),
+        GL::PixelFormat::RGB, GL::PixelType::UnsignedShort, {1, 3}, Containers::Array<char>{data, 3*6}, &stateOld};
+    ImageData2D b{std::move(a), &stateNew};
+
+    CORRADE_COMPARE(a.data(), nullptr);
+    CORRADE_COMPARE(a.size(), Vector2i{});
+
+    CORRADE_VERIFY(!b.isCompressed());
+    CORRADE_COMPARE(b.storage().alignment(), 1);
+    CORRADE_COMPARE(b.format(), pixelFormatWrap(GL::PixelFormat::RGB));
+    CORRADE_COMPARE(b.formatExtra(), 1337);
+    CORRADE_COMPARE(b.pixelSize(), 6);
+    CORRADE_COMPARE(b.size(), (Vector2i{1, 3}));
+    CORRADE_COMPARE(b.data(), data);
+    CORRADE_COMPARE(b.data().size(), 3*6);
+    CORRADE_COMPARE(b.importerState(), &stateNew);
+}
+
+void ImageDataTest::constructMoveCompressedAttachState() {
+    auto data = new char[8];
+    int stateOld, stateNew;
+    ImageData2D a{
+        CompressedPixelStorage{}.setCompressedBlockSize(Vector3i{4}),
+        GL::CompressedPixelFormat::RGBS3tcDxt1, {4, 4}, Containers::Array<char>{data, 8}, &stateOld};
+    ImageData2D b{std::move(a), &stateNew};
+
+    CORRADE_COMPARE(a.data(), nullptr);
+    CORRADE_COMPARE(a.size(), Vector2i{});
+
+    CORRADE_VERIFY(b.isCompressed());
+    CORRADE_COMPARE(b.compressedStorage().compressedBlockSize(), Vector3i{4});
+    CORRADE_COMPARE(b.compressedFormat(), compressedPixelFormatWrap(GL::CompressedPixelFormat::RGBS3tcDxt1));
+    CORRADE_COMPARE(b.size(), (Vector2i{4, 4}));
+    CORRADE_COMPARE(b.data(), data);
+    CORRADE_COMPARE(b.data().size(), 8);
+    CORRADE_COMPARE(b.importerState(), &stateNew);
+}
+
 void ImageDataTest::toViewGeneric() {
     auto data = new char[3*4];
     const ImageData2D a{PixelStorage{}.setAlignment(1),