The websocket code has internal buffers that expect data
not to be larger than UPDATE_BUF_SIZE
Modify rfbWriteExact() to split larger writes in multiple
frames.
Let webSocketsEncodeHybi() return -1 if somebody else
tries to pass a too large buffer to it.
(While in the libvncserver code rfbWriteExact() is the only
function that calls websocketEncode(), it is exposed by rfb.h,
so could theoretically also be called from application code)
The websocket code has internal buffers that expect data not to be larger than UPDATE_BUF_SIZE
Closes #484