RealARGBColorConverter: fix memory leak when calling ClassCopyProvider

This line causes BDV to keep a reference to the first (or more) images
shown. ClassCopyProvider keeps the keys used for creating class copies.
In the key was a ImgLib2 pixel. So class copy kept a reference to the
pixel. The pixel kept a reference to it's image, which than never gets
garbage collected.

src/main/java/net/imglib2/display/RealARGBColorConverter.java

@@ -57,7 +57,7 @@ class Instances
 					provider = new ClassCopyProvider<>( Imp.class, RealARGBColorConverter.class, double.class, double.class );
 			}
 		}
-		return provider.newInstanceForKey( type, min, max );
+		return provider.newInstanceForKey( type.getClass(), min, max );
 	}
 
 	public static class Imp< R extends RealType< ? > > implements RealARGBColorConverter< R >