noVNC was never processing more than one framebufferUpdate message per
onmessage event. If noVNC receives an incomplete framebufferUpdate and
then receives the rest of the framebufferUpdate plus another complete
framebufferUpdate, then it will fall permanently behind.

If there is more to process after a completed framebufferUpdate, then
execute normal_msg again.

All the render routines must return false if there is not enough data
in the receive queue to process their current update, and true
otherwise.