Fix #136906: Crash if asset library path contains back slash on macOS

Convert all slashes to native format when initializing an asset library. This
might convert slashes that are valid parts of the file name, but this just
leads to an error about a not found asset library, which is better than
crashing. This is a typical tradeoff when dealing with cross platform paths.

source/blender/asset_system/intern/utils.cc

@@ -25,6 +25,7 @@ std::string normalize_directory_path(StringRef directory)
               directory.data(),
               /* + 1 for null terminator. */
               std::min(directory.size() + 1, int64_t(sizeof(dir_normalized))));
+  BLI_path_slash_native(dir_normalized);
   BLI_path_normalize_dir(dir_normalized, sizeof(dir_normalized));
   return std::string(dir_normalized);
 }

source/blender/asset_system/intern/utils.hh

@@ -14,7 +14,7 @@ namespace blender::asset_system::utils {
 
 /**
  * Returns a normalized directory path with a trailing slash, and a maximum length of #PATH_MAX.
- * Slashes are not converted to native format (they probably should be though?).
+ * Slashes are converted to native format.
  */
 std::string normalize_directory_path(StringRef directory);