imageconverter: restore the --converter raw option.

Broken with 5a8d31bf68. Tests. Did I
mention I need tests for this?

src/Magnum/Trade/imageconverter.cpp

@@ -965,9 +965,10 @@ no -C / --converter is specified, AnyImageConverter is used.)")
             (outputDimensions == 2 && outputImages2D.front().isCompressed()) ||
             (outputDimensions == 3 && outputImages3D.front().isCompressed());
 
-        /* Load converter plugin */
+        /* Load converter plugin if a raw conversion is not requested */
-        Containers::Pointer<Trade::AbstractImageConverter> converter = converterManager.loadAndInstantiate(converterName);
+        Containers::Pointer<Trade::AbstractImageConverter> converter;
-        if(!converter) {
+        if(converterName != "raw"_s) {
+            if(!(converter = converterManager.loadAndInstantiate(converterName))) {
                 Debug{} << "Available converter plugins:" << ", "_s.join(converterManager.aliasList());
                 return 2;
             }
@@ -976,21 +977,24 @@ no -C / --converter is specified, AnyImageConverter is used.)")
             if(args.isSet("verbose")) converter->addFlags(Trade::ImageConverterFlag::Verbose);
             if(i < args.arrayValueCount("converter-options"))
                 Implementation::setOptions(*converter, "AnyImageConverter", args.arrayValue("converter-options", i));
+        }
 
-        /* This is the last --converter (or the implicit AnyImageConverter at
+        /* This is the last --converter (a raw output, a file-capable converter
-           the end), output to a file and exit the loop */
+           or the implicit AnyImageConverter at the end), output to a file and
-        if(i + 1 >= converterCount && (converter->features() & (
+           exit the loop */
+        if(i + 1 >= converterCount && (converterName == "raw"_s || (converter->features() & (
             Trade::ImageConverterFeature::Convert1DToFile|
             Trade::ImageConverterFeature::Convert2DToFile|
             Trade::ImageConverterFeature::Convert3DToFile|
             Trade::ImageConverterFeature::ConvertCompressed1DToFile|
             Trade::ImageConverterFeature::ConvertCompressed2DToFile|
-            Trade::ImageConverterFeature::ConvertCompressed3DToFile)))
+            Trade::ImageConverterFeature::ConvertCompressed3DToFile))))
         {
             /* Decide what converter feature we should look for for given
                dimension count. This has to be redone each iteration, as a
                converted could have converted an uncompressed image to a
                compressed one and vice versa. */
+            if(converterName != "raw"_s) {
                 Trade::ImageConverterFeatures expectedFeatures;
                 if(outputDimensions == 1) {
                     expectedFeatures = outputIsCompressed ?
@@ -1019,6 +1023,7 @@ no -C / --converter is specified, AnyImageConverter is used.)")
                     err << outputDimensions << Debug::nospace << "D image to file conversion, only" << converter->features();
                     return 6;
                 }
+            }
 
             if(args.isSet("verbose")) {
                 Debug d;
@@ -1103,6 +1108,11 @@ no -C / --converter is specified, AnyImageConverter is used.)")
         /* This is not the last converter, expect that it's capable of
            image-to-image conversion */
         } else {
+            if(converterName == "raw"_s) {
+                Error{} << "Only the very last --converter can be raw";
+                return 1;
+            }
+
             CORRADE_INTERNAL_ASSERT(i < converterCount);
             if(converterCount > 1 && args.isSet("verbose"))
                 Debug{} << "Processing (" << Debug::nospace << (i+1) << Debug::nospace << "/" << Debug::nospace << converterCount << Debug::nospace << ") with" << converterName << Debug::nospace << "...";