rework web server so multiple clients can be server at a time

backend/nodes/threshold_mask.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 import numpy as np
 from backend.node_registry import register_node
+from backend.execution_context import emit_preview
 from backend.data_types import DataField, encode_preview
 from backend.nodes.helpers import _mask_overlay
 
@@ -52,10 +53,7 @@ class ThresholdMask:
         else:
             mask = (data < t).astype(np.uint8) * 255
 
-        if ThresholdMask._broadcast_fn is not None:
-            overlay = _mask_overlay(field, mask)
-            ThresholdMask._broadcast_fn(
-                ThresholdMask._current_node_id, encode_preview(overlay),
-            )
+        overlay = _mask_overlay(field, mask)
+        emit_preview(encode_preview(overlay))
 
         return (mask,)