libweston: add color ops cap and bool renderer shadow buffer

This adds the libweston capability bit for "color operations" which
refers to a renderer's support for operations needed for color
management. GL-renderer will grow the support while Pixman-renderer will
not, which is why the cap is needed.

To make an example use of the cap, this also adds new API:
weston_output_set_renderer_shadow_buffer(). This is a temporary API to
enable future experimental features. The first such feature will be the
renderer internal shadow buffer, the boolean variable for it taken from
Harish Krupo's "weston.ini: introduce use-shadow-fbo in output config".

Obviously this patch does not implement the renderer shadow buffer. No
renderer sets WESTON_CAP_COLOR_OPS yet so trying to enable it will fail.

The documentation here is deliberately vague, because the bits needed
for color management will come in trickling for a long time until we can
call it color management in any sense. Until then, the temporary API
shall remain, perhaps poorly named.

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