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/shm/shm_fs.go

@@ -33,9 +33,7 @@ func ShmUnlink(name string) error {
 	if runtime.GOOS == "openbsd" {
 		return os.Remove(openbsd_shm_path(name))
 	}
-	if strings.HasPrefix(name, "/") {
-		name = name[1:]
-	}
+	name = strings.TrimPrefix(name, "/")
 	return os.Remove(filepath.Join(SHM_DIR, name))
 }