This adds a capture_screenshot request which returns an image of the
screen in the client-supplied wl_buffer for the given display output.
A 'done' event is used to indicate when screenshotting has finished and
the wl_buffer is ready to be read.
Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>
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