add option to CLI and pngwriter that allows user to set PNG compression level

- In CLI: the argument is --png_compression <0..9> (-z<0..9>)
- In API, pass `compress_level` to PngWriter.save_image_and_prompt_to_png()

Compression ranges from 0 (no compression) to 9 (maximum compression).
Default value is 6 (as specified by Pillow package).

This addresses an issue first raised in #652.

ldm/invoke/args.py

@@ -378,6 +378,14 @@ class Args(object):
             default='stable-diffusion-1.4',
             help='Indicates which diffusion model to load. (currently "stable-diffusion-1.4" (default) or "laion400m")',
         )
+        model_group.add_argument(
+            '--png_compression','-z',
+            type=int,
+            default=6,
+            choices=range(0,9),
+            dest='png_compression',
+            help='level of PNG compression, from 0 (none) to 9 (maximum). Default is 6.'
+        )
         model_group.add_argument(
             '--sampler',
             '-A',
@@ -649,6 +657,14 @@ class Args(object):
             dest='save_intermediates',
             help='Save every nth intermediate image into an "intermediates" directory within the output directory'
         )
+        render_group.add_argument(
+            '--png_compression','-z',
+            type=int,
+            default=6,
+            choices=range(0,10),
+            dest='png_compression',
+            help='level of PNG compression, from 0 (none) to 9 (maximum). Default is 6.'
+        )
         img2img_group.add_argument(
             '-I',
             '--init_img',

ldm/invoke/pngwriter.py

@@ -33,13 +33,13 @@ class PngWriter:
 
     # saves image named _image_ to outdir/name, writing metadata from prompt
     # returns full path of output
-    def save_image_and_prompt_to_png(self, image, dream_prompt, name, metadata=None):
+    def save_image_and_prompt_to_png(self, image, dream_prompt, name, metadata=None, compress_level=6):
         path = os.path.join(self.outdir, name)
         info = PngImagePlugin.PngInfo()
         info.add_text('Dream', dream_prompt)
         if metadata:
           info.add_text('sd-metadata', json.dumps(metadata))
-        image.save(path, 'PNG', pnginfo=info)
+        image.save(path, 'PNG', pnginfo=info, compress_level=compress_level)
         return path
 
     def retrieve_metadata(self,img_basename):

ldm/invoke/readline.py

@@ -53,6 +53,7 @@ COMMANDS = (
     '--log_tokenization','-t',
     '--hires_fix',
     '--inpaint_replace','-r',
+    '--png_compression','-z',
     '!fix','!fetch','!history','!search','!clear',
     '!models','!switch','!import_model','!edit_model'
     )