Switch over to the new imaging backend for icat

Greatly simplifies a whole bunch of code. The new backend takes care of
falling back to ImageMagick efficiently itself.

tools/utils/images/serialize_test.go

@@ -12,7 +12,7 @@ import (
 var _ = fmt.Print
 
 func TestImageSerialize(t *testing.T) {
-	img, err := OpenNativeImageFromReader(bytes.NewReader(kitty.KittyLogoAsPNGData))
+	img, _, err := OpenImageFromReader(bytes.NewReader(kitty.KittyLogoAsPNGData))
 	if err != nil {
 		t.Fatal(err)
 	}