renderdiff: add two params to local_test.sh (#9301)

- --no_rebuild will skip building the gltf_viewer again
 - --num_threads will limit the number of threads used in running
   the rendering test.

Both of these options are meant for debugging locally.

test/renderdiff/src/render.py

@@ -83,7 +83,8 @@ def _render_test_config(gltf_viewer,
             local_only=False,
             opengl_lib=None,
             vk_icd=None,
-            test_filter=None):
+            test_filter=None,
+            num_threads=None):
   assert os.path.isdir(output_dir), f"output directory {output_dir} does not exist"
   assert os.access(gltf_viewer, os.X_OK)
 
@@ -93,7 +94,7 @@ def _render_test_config(gltf_viewer,
   gltf_viewer_abs = os.path.abspath(gltf_viewer)
 
   results = []
-  with concurrent.futures.ThreadPoolExecutor() as executor:
+  with concurrent.futures.ThreadPoolExecutor(max_workers=num_threads) as executor:
     futures = []
     for test in test_config.tests:
       test_json_path = os.path.abspath(
@@ -130,6 +131,7 @@ if __name__ == "__main__":
   parser.add_argument('--opengl_lib', help='Path to the folder containing OpenGL driver lib (for LD_LIBRARY_PATH)')
   parser.add_argument('--vk_icd', help='Path to VK ICD file')
   parser.add_argument('--test_filter', help='Filter for the tests to run')
+  parser.add_argument('--num_threads', help='Number of threads to use for rendering', type=int)
 
   args, _ = parser.parse_known_args(sys.argv[1:])
   test = test_config.parse_from_path(args.test)
@@ -140,7 +142,8 @@ if __name__ == "__main__":
                         args.output_dir,
                         opengl_lib=args.opengl_lib,
                         vk_icd=args.vk_icd,
-                        test_filter=args.test_filter)
+                        test_filter=args.test_filter,
+                        num_threads=args.num_threads)
 
   with open(f'{output_dir}/render_results.json', 'w') as f:
     f.write(json.dumps(results, indent=2))