add metadata to scn2img intermediate image output

Following metadata is written:
- "prompt" contains the representation of the SceneObject corresponding to the intermediate image
- "seed" contains the seed at the start of the function that generated this intermediate image
- "width" and "height" contain the size of the image.

To get the seed at the start of the render function without using it, a class SeedGenerator is added and
used instead of the python generator functions.

Fixes warning thrown in console: "> Couldn't find metadata on image", originally reported by @codedealer in Sygil-Dev#1179 (review)

scripts/scn2img.py

@@ -3,7 +3,6 @@
 import collections
 import yaml
 import math
-import copy
 import random
 from typing import List, Union, Dict, Callable, Any, Optional, Type, Tuple