enhance outcropping with ability to direct contents of new regions

- When outcropping an image you can now add a `--new_prompt` option, to specify
  a new prompt to be used instead of the original one used to generate the image.

- Similarly you can provide a new seed using `--seed` (or `-S`). A seed of zero
  will pick one randomly.

- This PR also fixes the crash that happened when trying to outcrop an image
  that does not contain InvokeAI metadata.

ldm/generate.py

@@ -561,18 +561,22 @@ class Generate:
             ):
         # retrieve the seed from the image;
         seed   = None
-        image_metadata = None
         prompt = None
 
         args   = metadata_from_png(image_path)
-        seed   = args.seed
-        prompt = args.prompt or ''
-        if seed == 0:
-            seed = random.randrange(0, np.iinfo(np.uint32).max)
-            opt.seed = seed
-            print(f'>> generated new seed {seed} and prompt "{prompt}" for {image_path}')
+        if opt.seed is not None:
+            seed = opt.seed
+        elif args.seed >= 0:
+            seed = args.seed
         else:
-            print(f'>> retrieved seed {seed} and prompt "{prompt}" from {image_path}')
+            seed   = random.randrange(0, np.iinfo(np.uint32).max)
+
+        if opt.prompt is not None:
+            prompt = opt.prompt
+        else:
+            prompt = args.prompt
+
+        print(f'>> using seed {seed} and prompt "{prompt}" for {image_path}')
 
         # try to reuse the same filename prefix as the original file.
         # we take everything up to the first period
@@ -619,6 +623,10 @@ class Generate:
                     extend_instructions[direction]=int(pixels)
                 except ValueError:
                     print(f'** invalid extension instruction. Use <directions> <pixels>..., as in "top 64 left 128 right 64 bottom 64"')
+
+            opt.seed = seed
+            opt.prompt = prompt
+            
             if len(extend_instructions)>0:
                 restorer = Outcrop(image,self,)
                 return restorer.process (