screen-share: Permit screen sharing even without pointer

In case weston has no pointer and does have a keyboard, starting the screen sharing via 'Control-Alt-S' from the keyboard fails with the following message in weston log:

Cannot pick output: Seat does not have pointer

This is because weston attempts to determine which output to share based on the position of pointer, the output where the pointer is located is shared.

In case there is no pointer, the output cannot be determined. Define this case such that, if there is no pointer, share all outputs instead of failing outright.