Add font containers for global and custom fonts

All global fonts and all custom fonts in a level are now loaded, and
added to their respective "vectors". The selected font is still always
as the global font.png, and the custom level font also isn't selected
yet, but it's now easier to implement that.

Also, I added FILESYSTEM_enumerateAssets, which #902 already has but I
needed it now. I also rewrote it to not use std::vector<std::string>.
That was my idea, it's also how FILESYSTEM_getLanguageCodes worked,
so for symmetry, that function is getting changed as well.

desktop_version/src/FileSystemUtils.cpp

@@ -1,3 +1,5 @@
+#include "FileSystemUtils.h"
+
 #include <physfs.h>
 #include <SDL.h>
 #include <stdarg.h>
@@ -521,8 +523,6 @@ void FILESYSTEM_loadZip(const char* filename)
     }
 }
 
-void FILESYSTEM_unmountAssets(void);
-
 bool FILESYSTEM_mountAssets(const char* path)
 {
     const char* real_dir = PHYSFS_getRealDir(path);
@@ -1042,26 +1042,96 @@ void FILESYSTEM_enumerateLevelDirFileNames(
     }
 }
 
-std::vector<std::string> FILESYSTEM_getLanguageCodes(void)
+const char* FILESYSTEM_enumerate(const char* folder, EnumHandle* handle)
 {
-    std::vector<std::string> list;
-    char** fileList = PHYSFS_enumerateFiles("lang");
-    char** item;
+    /* List all files in a folder with PHYSFS_enumerateFiles.
+     *
+     * Doing it this way means we can decide and filter
+     * what's in the lists (in wrapper functions).
+     *
+     * Called like this:
+     *
+     *  EnumHandle handle = {};
+     *  const char* item;
+     *  while ((item = FILESYSTEM_enumerate("graphics", &handle)) != NULL)
+     *  {
+     *      puts(item);
+     *  }
+     *  FILESYSTEM_freeEnumerate(&handle);
+     */
 
-    for (item = fileList; *item != NULL; item++)
+    if (handle->physfs_list == NULL)
     {
-        char fullName[128];
-        SDL_snprintf(fullName, sizeof(fullName), "lang/%s", *item);
+        // First iteration, set things up
+        handle->physfs_list = PHYSFS_enumerateFiles(folder);
+        handle->_item = handle->physfs_list;
+    }
 
-        if (FILESYSTEM_isDirectory(fullName) && *item[0] != '.')
+    /* Return the next item, and increment the pointer.
+     * (once we return NULL, handle->_item points to 1 past end of array) */
+    return *(handle->_item++);
+}
+
+const char* FILESYSTEM_enumerateAssets(const char* folder, EnumHandle* handle)
+{
+    /* This function enumerates ONLY level-specific assets.
+     * If there are only global assets and no level-specific ones,
+     * we want an empty list.
+     *
+     * This function is called the same way as FILESYSTEM_enumerate, see above. */
+
+    if (!FILESYSTEM_isAssetMounted(folder))
+    {
+        return NULL;
+    }
+
+    char mounted_path[MAX_PATH];
+    getMountedPath(mounted_path, sizeof(mounted_path), folder);
+
+    const char* item;
+    while ((item = FILESYSTEM_enumerate(mounted_path, handle)) != NULL)
+    {
+        char full_name[128];
+        SDL_snprintf(full_name, sizeof(full_name), "%s/%s", mounted_path, item);
+        if (FILESYSTEM_isFile(full_name) && item[0] != '.')
         {
-            list.push_back(*item);
+            return item;
         }
     }
 
-    PHYSFS_freeList(fileList);
+    return NULL;
+}
 
-    return list;
+const char* FILESYSTEM_enumerateLanguageCodes(EnumHandle* handle)
+{
+    /* This function enumerates all the language codes.
+     *
+     * This function is called the same way as FILESYSTEM_enumerate, see above. */
+
+    const char* item;
+    while ((item = FILESYSTEM_enumerate("lang", handle)) != NULL)
+    {
+        char full_name[128];
+        SDL_snprintf(full_name, sizeof(full_name), "lang/%s", item);
+
+        if (FILESYSTEM_isDirectory(full_name) && item[0] != '.')
+        {
+            return item;
+        }
+    }
+
+    return NULL;
+}
+
+void FILESYSTEM_freeEnumerate(EnumHandle* handle)
+{
+    /* Call this function after enumerating with FILESYSTEM_enumerate or friends. */
+    if (handle == NULL)
+    {
+        return;
+    }
+
+    PHYSFS_freeList(handle->physfs_list);
 }
 
 static int PLATFORM_getOSDirectory(char* output, const size_t output_size)