Trade: add docs about multi-level image conversion.

doc/snippets/MagnumTrade.cpp

@@ -90,6 +90,21 @@ if(!converter || !converter->convertToFile(image, "image.png"))
 /* [AbstractImageConverter-usage-file] */
 }
 
+{
+/* [AbstractImageConverter-usage-file-levels] */
+PluginManager::Manager<Trade::AbstractImageConverter> manager;
+Containers::Pointer<Trade::AbstractImageConverter> converter =
+    manager.loadAndInstantiate("AnyImageConverter");
+
+Image2D level0{PixelFormat::RGBA16F, {256, 256}, DOXYGEN_IGNORE({})};
+Image2D level1{PixelFormat::RGBA16F, {128, 128}, DOXYGEN_IGNORE({})};
+Image2D level2{PixelFormat::RGBA16F, {64, 64}, DOXYGEN_IGNORE({})};
+
+if(!converter || !converter->convertToFile({level0, level1, level2}, "image.exr"))
+    Fatal{} << "Can't save image.exr with AnyImageConverter";
+/* [AbstractImageConverter-usage-file-levels] */
+}
+
 {
 Image2D image{{}, {}, {}};
 /* [AbstractImageConverter-usage-image] */

src/Magnum/Trade/AbstractImageConverter.h

@@ -495,6 +495,18 @@ image converter plugins.
     exposing functionality of all image converter plugins on a command line as
     well as performing introspection of image files.
 
+@subsection Trade-AbstractImageConverter-usage-file-levels Saving a set of mip levels
+
+Certain file formats, such as OpenEXR, DDS or ICO, are capable of storing
+multiple image mip levels in a single file. Each format has a slightly
+different set of rules for image sizes and their order, in general putting the
+largest level first and then gradually halving the size should always work. In
+the following snippet we'll save three mip levels to an EXR file, again using
+the @ref AnyImageConverter plugin, which will then most likely delegate to
+@link OpenExrImageConverter @endlink:
+
+@snippet MagnumTrade.cpp AbstractImageConverter-usage-file-levels
+
 @subsection Trade-AbstractImageConverter-usage-image Converting image data
 
 In the following snippet we use @ref StbDxtImageConverter to convert the same