Scale-crop mode scales the wallpaper to tightly fill the whole output,
but preserving wallpaper aspect ratio. If aspect ratio differs from the
output's, the wallpaper is centered cutting it from top/bottom or
left/right.
Add this to the weston.ini man page, and explain all three modes.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>