twn_textures.c: don't zero fill intermediate surfaces

src/twn_textures.c

@@ -213,9 +213,6 @@ static void recreate_current_atlas_texture(TextureCache *cache) {
     /*       for example, if full page of 64x64 tiles was already filled, there's no real reason to process them further */
     SDL_Surface *atlas_surface = cache->atlas_surfaces[cache->atlas_index];
 
-    /* clear */
-    SDL_FillRect(atlas_surface, NULL, 0);
-
     /* blit the texture surfaces onto the atlas */
     for (size_t i = 0; i < shlenu(cache->hash); ++i) {
         /* skip all that aren't part of currently built one */