Do the right thing with empty directory path.

When we convert a file that's in our CWD, on Unix the folder component
of the path will simply be "", whereas opendir() wants ".".

I want to take another more substantial pass at texture resolution, once
we're out of urgent bugfix mode.

src/utils/File_Utils.cpp

@@ -102,7 +102,7 @@ namespace FileUtils {
     {
         std::vector<std::string> fileList;
 #if defined( __unix__ ) || defined( __APPLE__ )
-        DIR *dir = opendir(folder);
+        DIR *dir = opendir(strlen(folder) > 0 ? folder : ".");
         if (dir != nullptr) {
             for (;;) {
                 struct dirent *dp = readdir(dir);