protocol: fix readonly handling (#373)

protocol: fix readonly handling (#373)

Previously, if running in readonly mode (`-R`), we would keep
accumulating incoming websocket data to `pss->buffer`, resizing it with
every incoming message. This would also break the `RESIZE` operation as
we would never get rid of the `INPUT` message.

Now we discard `INPUT` messages in `readonly` mode, 1) allowing resize
to work and 2) preventing the buffer from growing indefinitely.