Store the user-defined monitors in the RandR screen private.

Generate a list of monitors from both the user-defined ones and from
any outputs not mentioned in one of the user-defined monitors. This list
covers both the outputs in the main screen as well as any slaves.

v1.1: airlied: fix up primary skipping bug,
fix wrong height initialiser
add get_active flag from updated protocol.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>