Make the sample script a little nicer

docs/graphics-protocol.rst

@@ -101,7 +101,8 @@ features of the graphics protocol:
    import sys
    from base64 import standard_b64encode
 
-   def serialize_gr_command(cmd, payload=None):
+   def serialize_gr_command(**cmd):
+      payload = cmd.pop('payload', None)
       cmd = ','.join('{}={}'.format(k, v) for k, v in cmd.items())
       ans = []
       w = ans.append
@@ -112,18 +113,17 @@ features of the graphics protocol:
       w(b'\033\\')
       return b''.join(ans)
 
-   def write_chunked(cmd, data):
-      data = standard_b64encode(data)
+   def write_chunked(**cmd):
+      data = standard_b64encode(cmd.pop('data'))
       while data:
          chunk, data = data[:4096], data[4096:]
          m = 1 if data else 0
-         cmd['m'] = m
-         sys.stdout.buffer.write(serialize_gr_command(cmd, chunk))
+         sys.stdout.buffer.write(serialize_gr_command(payload=chunk, m=m, **cmd))
          sys.stdout.flush()
          cmd.clear()
 
    with open(sys.argv[-1], 'rb') as f:
-      write_chunked({'a': 'T', 'f': 100}, f.read())
+      write_chunked(a='T', f=100, data=f.read())
 
 
 Save this script as :file:`png.py`, then you can use it to display any PNG