backend-rdp: allow to force compression off

By default the client communicates its preference with regards to
compression to the server. However, some clients always use
compression, which is not ideal for certain environments (e.g.
low performance embedded devices in a local network with plenty
of bandwidth). Allow to disable compression server-side which will
override the clients request for compression.

Signed-off-by: Stefan Agner <stefan@agner.ch>