Work in progress
Add RRE encoding and use it for empty space

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
framebuffer: send at most one non-empty response to every incremental update request

We should avoid sending empty updates (zero-length rectangle list)
because it seems to cause some clients to stop asking for more
updates.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>