noVNC: Disable COPYRECT and prefer TIGHT_PNG

There seems to be an unresolved screen corruption issue with the Qemu
VNC COPYRECT encoding implementation. [1] Comment it out from the
client's preferred encoding list and, since Qemu VNC supports it, always
prefer TIGHT_PNG over TIGHT, for performance reasons.

[1] http://patchwork.ozlabs.org/patch/32165/