sdcv: Don't walk into "res/" subdirectories

When searching for .ifo files use the same optimization as we do in the
koreader frontend in getIfosInDir(path) in readerdictionary.lua.

Fortunately sdcv uses the for_each_file function exclusively to search
for .ifo files, making the patch trivial.

I have several dictionaries on my Kobo Aura One, some of which contain
'res' subdirectories with many files. After a restart or cold boot, the
first dictionary lookup would take around 45s, while subsequent lookups
would only take merely around 0.5s. My theory for this massive
difference is that after reboot, the file system cache is empty, and
therefore the file system structure for all the resource directories
must first be loaded from the flash, which seems to be quite slow.

With the patched sdcv there is now no difference between the first and
the following lookups anymore. In addition, the average lookup time on
my device drops from 0.5s to about 0.3s.

thirdparty/sdcv/CMakeLists.txt

@@ -6,6 +6,8 @@ list(APPEND PATCH_FILES
     sdcv-locale-hack.patch
     # Fix compilation with newer GLib.
     compat_with_newer_glib.patch
+    # Don't walk into "res/" subdirectories.
+    sdcv-ignore-res.patch
 )
 
 string(APPEND GLIB2_INCLUDE_DIRS

thirdparty/sdcv/sdcv-ignore-res.patch

@@ -0,0 +1,19 @@
+diff --git a/src/utils.cpp b/src/utils.cpp
+index 33bfeaac..a1fbdf1e 100644
+--- a/src/utils.cpp
++++ b/src/utils.cpp
+@@ -66,7 +66,13 @@ static void __for_each_file(const std::string &dirname, const std::string &suff,
+         while ((filename = g_dir_read_name(dir)) != nullptr) {
+             const std::string fullfilename(dirname + G_DIR_SEPARATOR_S + filename);
+             if (g_file_test(fullfilename.c_str(), G_FILE_TEST_IS_DIR))
+-                __for_each_file(fullfilename, suff, order_list, disable_list, f);
++                {
++                    // Don't walk into "res/" subdirectories, as per Stardict specs, they
++                    // may contain possibly many resource files (image, audio files...)
++                    // that could slow down our walk here.
++                    if (strcmp(filename, "res"))
++                        __for_each_file(fullfilename, suff, order_list, disable_list, f);
++                }
+             else if (g_str_has_suffix(filename, suff.c_str()) && std::find(order_list.begin(), order_list.end(), fullfilename) == order_list.end()) {
+                 const bool disable = std::find(disable_list.begin(),
+                                                disable_list.end(),