[wip] Trade: rebuild AbstractImageConverter around ImageData.

And not views, as ImageData contains extra metadata. Currently just the
importer state, but eventually that will be changed to a generic
key/value store for stuff like color correction, EXIF, gamma, etc., and
if the plugin interfaces would consume only views (which don't and won't
have such key/value data), the data would get lost.

A similar approach is being taken with the AbstractSceneConverter.

TODO: implement this to be more than just a TODO

src/Magnum/Trade/AbstractImageConverter.cpp

@@ -272,6 +272,8 @@ Containers::Optional<ImageData3D> AbstractImageConverter::convert(const ImageDat
 #ifndef CORRADE_NO_ASSERT
 namespace {
 
+/** @todo reuse from AbstractSceneConverter when both take ImageData and not
+    views */
 template<UnsignedInt dimensions, template<UnsignedInt, class> class View> bool checkImageValidity(const char* const messagePrefix, const View<dimensions, const char>& image) {
     /* At some point there might be a file format that allows zero-sized
        images, but so far I don't know about any. When such format appears,
@@ -539,6 +541,8 @@ AbstractImageConverter::convertToData(const ImageData3D& image) {
 #ifndef CORRADE_NO_ASSERT
 namespace {
 
+/** @todo reuse from AbstractSceneConverter when both take ImageData and not
+    views */
 template<UnsignedInt dimensions> bool checkImageValidity(const char* const messagePrefix, const Containers::ArrayView<const BasicImageView<dimensions>> imageLevels) {
     CORRADE_ASSERT(!imageLevels.isEmpty(),
         messagePrefix << "at least one image has to be specified", false);
@@ -562,6 +566,8 @@ template<UnsignedInt dimensions> bool checkImageValidity(const char* const messa
     return true;
 }
 
+/** @todo reuse from AbstractSceneConverter when both take ImageData and not
+    views */
 template<UnsignedInt dimensions> bool checkImageValidity(const char* const messagePrefix, const Containers::ArrayView<const BasicCompressedImageView<dimensions>> imageLevels) {
     CORRADE_ASSERT(!imageLevels.isEmpty(),
         messagePrefix << "at least one image has to be specified", false);

src/Magnum/Trade/AbstractImageConverter.h

@@ -1015,6 +1015,8 @@ class MAGNUM_TRADE_EXPORT AbstractImageConverter: public PluginManager::Abstract
         #endif
         convertToData(const CompressedImageView3D& image);
 
+        /** @todo with extras, this all needs to take a Data */
+
         /**
          * @brief Convert a 1D image data to a raw data
          * @m_since_latest