rpi-config,README: Add option to enable offline image compositing

Set this variable to enable support for dispmanx offline compositing
DISMANX_OFFLINE = "1"

This will enable the firmware to fall back to off-line compositing of
Dispmanx elements. Normally the compositing is done on-line, during scanout,
but cannot handle too many elements. With off-line enabled, an off-screen
buffer is allocated for compositing. When scene complexity (number and sizes
of elements) is high, compositing will happen off-line into the buffer.

Heavily recommended for Wayland/Weston.

See: http://wayland.freedesktop.org/raspberrypi.html

Change-Id: I722b20ff5bdba1bdd0683d30bee07c3934c95a1f
Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>