Basically if the calculator sends two messages in a row, it is possible
for a single libusb_handle_events() to get both. And the comm structure
wasn't designed for that, because it could buffer only one message at a
time, which the user needed to read after event handling.
The comm structure now has a 16-message buffer, which should be more
than enough for any single event handling loop. On the user level this
has implications in that fxlink_device_finish_bulk_IN() must be called
*in a loop* after each event handling cycle.
Reported in https://git.planet-casio.com/Lephenixnoir/gint/pulls/27
