Missed two places where PngWriter's __init__() signature changed

[xaroth8088]

@lstein This appears to be related to recent work removing the batch_size parameter from PngWriter, and is just two spots that were missed (lines 91 and 134).

Exception happened during processing of request from ('127.0.0.1', 57222)
Traceback (most recent call last):
  File "/home/xaroth/anaconda3/envs/lstein-ldm/lib/python3.8/socketserver.py", line 650, in process_request_thread
    self.finish_request(request, client_address)
  File "/home/xaroth/anaconda3/envs/lstein-ldm/lib/python3.8/socketserver.py", line 360, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/home/xaroth/anaconda3/envs/lstein-ldm/lib/python3.8/socketserver.py", line 720, in __init__
    self.handle()
  File "/home/xaroth/anaconda3/envs/lstein-ldm/lib/python3.8/http/server.py", line 427, in handle
    self.handle_one_request()
  File "/home/xaroth/anaconda3/envs/lstein-ldm/lib/python3.8/http/server.py", line 415, in handle_one_request
    method()
  File "/home/xaroth/lstein-stable-diffusion/stable-diffusion/ldm/dream/server.py", line 91, in do_POST
    pngwriter = PngWriter(
TypeError: __init__() takes from 2 to 3 positional arguments but 4 were given

[bakkot]

#231 should fix this.