Limit concurrent thumbnail downloads to 8

src/python/image_provider.py

@@ -20,6 +20,8 @@ from nio.api import ResizingMethod
 Size      = Tuple[int, int]
 ImageData = Tuple[bytearray, Size, int]  # last int: pyotherside format enum
 
+CONCURRENT_DOWNLOADS_LIMIT = asyncio.BoundedSemaphore(8)
+
 
 @dataclass
 class Thumbnail:
@@ -85,6 +87,7 @@ class Thumbnail:
         )
         parsed = urlparse(self.mxc)
 
+        async with CONCURRENT_DOWNLOADS_LIMIT:
             response = await client.thumbnail(
                 server_name = parsed.netloc,
                 media_id    = parsed.path.lstrip("/"),