man: add description for specifying modes with aspect-ratio in weston.ini

man: add description for specifying modes with aspect-ratio in weston.ini

This patch adds information about the new resolution-format that can
be specified by a user in weston.ini to select a CEA mode. CEA defines
timing of a video mode, which is considered as a standard for
HDMI certification and compliance testing. It defines each and every
parameter, of  a video mode, like h/vactive,h/vfront h/vback etc.,
including aspect-ratio information. The drm layer, specifies the
aspect-ratio information in user-mode flag bits 19-22. For the non-CEA
modes a value of 0 is given in the aspect-ratio flag bits. Each
CEA-mode is identified by a unique, Video Identification Code (VIC).
For example, VIC=4 is 1280x720@60 aspect-ratio 16:9.
This mode will be different than a non-CEA mode 1280x720@60 0:0.

The new mode-format helps to differentiate between the CEA and
non-CEA modes, by letting user specify aspect-ratio along with other
paremeters: mode=widthxheight@rr ratio.

This helps when certification testing is done, in tests like 7-27,
the HDMI analyzer applies a particular CEA mode, and expects the
applied mode to be with exactly same timings, including the
aspect-ratio and VIC field.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>