clients/screenshot: Allow weston-screenshooter to be called directly

As is stands now, libexec/weston-screenshooter can only be called from
within weston server. This will allow weston-screenshooter to be called
on the command line. The final scope here is to allow taking screenshots
without the need of a keyboard.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>