Rework the Destination filesystem to make --renderStaticToDisk work

See #9626

hugofs/hashing_fs.go

@@ -22,7 +22,10 @@ import (
 	"github.com/spf13/afero"
 )
 
-var _ afero.Fs = (*md5HashingFs)(nil)
+var (
+	_ afero.Fs            = (*md5HashingFs)(nil)
+	_ FilesystemUnwrapper = (*md5HashingFs)(nil)
+)
 
 // FileHashReceiver will receive the filename an the content's MD5 sum on file close.
 type FileHashReceiver interface {
@@ -45,6 +48,10 @@ func NewHashingFs(delegate afero.Fs, hashReceiver FileHashReceiver) afero.Fs {
 	return &md5HashingFs{Fs: delegate, hashReceiver: hashReceiver}
 }
 
+func (fs *md5HashingFs) UnwrapFilesystem() afero.Fs {
+	return fs.Fs
+}
+
 func (fs *md5HashingFs) Create(name string) (afero.File, error) {
 	f, err := fs.Fs.Create(name)
 	if err == nil {