Instead of dropping the connection, we now implement a simple static
colour map that emulates RGB332.
The quality isn't great, but it's better than dropping the connection
without any explanation.
Implement minimal support for ExtendedDesktopSize pseudo-encoding
and SetDesktopSize client message.
The opaque nvnc_desktop_layout structure contains all information
from the SetDesktopSize client message.
The encoding identifiers are supposed to be interpreted as a range. I
interpreted the upper and lower limits as two discrete quality settings
instead, which is wrong.