python-codecs: add Pillow fallback

plugins/python-codecs/src/thread.py

@@ -0,0 +1,10 @@
+import asyncio
+from typing import Any
+import concurrent.futures
+
+# vips is already multithreaded, but needs to be kicked off the python asyncio thread.
+toThreadExecutor = concurrent.futures.ThreadPoolExecutor(max_workers=2, thread_name_prefix="image")
+
+async def to_thread(f):
+    loop = asyncio.get_running_loop()
+    return await loop.run_in_executor(toThreadExecutor, f)