fix: wrong offset when memoryview format is non-byte

the offset should be in bytes, not elements. See also bokeh/ipywidgets_bokeh#46 (comment)

panel/io/ipywidget.py

@@ -28,7 +28,7 @@ def send(self, stream, msg_type, content=None, parent=None, ident=None, buffers=
             offsets = [start]
 
             for buffer in buffers[:-1]:
-                start += len(buffer)
+                start += memoryview(buffer).nbytes
                 offsets.append(start)
 
             u32 = lambda n: n.to_bytes(4, "big")