compositor: add weston.ini option use-renderer-shadow

This adds an output section option use-renderer-shadow in weston.ini.
This option is only recognized with headless and DRM backends, because
it requires GL-renderer and does not support resizing outputs.

The option is called use-renderer-shadow because this is what it does
right now. In the future the same setting will be used to turn on more
complex image processing when operational pieces required for color
management land. Once color management is implemented, this option is
expected to be removed. This option allows developer testing of features
to be used to implement color management.

This is a rewrite of "weston.ini: introduce use-shadow-fbo in output
config" by Harish Krupo. The main.c code is structured differently, the
weston.ini option is renamed, and the man page paragraph is moved to
weston.ini.man with different content.

Cc: Harish Krupo <harishkrupo@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>