rfb.js: rQwait, cuttext simplification.

- rQwait checks the receive queue to see if there is enough data to
  satisfy the following request. If not it returns true (which is
  almost always translated into an immediate return false by the
  caller).

- rQwait is called quite a bit and this generally allows 4 lines to
  become 1 line where it is called.

- rQwait allows simplification of cuttext processing. No global
  tracking needed anymore.

Overall, about 60 lines less code.