Fix low quality connections

If you try to create a VNC remote connection with the "low quality" setting, only a blue or black screen is shown. The reason is libvnc which does not support tight encoding and 8 bits per pixel at the same time. Only 8 bpp without tight encoding or tight encoding with 16 or 32 bpp is possible.

This patch adds an option to switch to 8 bpp without tight encoding and sets the default for low quality connections to tight encoding at 16 bpp.