Weston debug screenshot keyboardless

This series is an attempt to enable weston debug to take screenshots without the need of a keyboard.

The caveat here is that we're re-using the weston screenshot interface used by clients/screenshot and giving that this is re-scheduling a repaint would (would potentially) remove an artifact seen on the screen.

This series exposes the screenshot functions and allows weston debug to re-use them. We add a new debug scope named screenshot. We verify if the debug interface is enabled when calling the shoot request not at binding so it would allow other clients to use (as the screenshot debug scope will no available at binding time).

This initially added a dummy screenshot debug scope which was not really needed, in the new version the we create a simple library libhshared_screenshot which the client/screenshot can be used to link against. Unfortunately the tests can't really make uses of it as it is now. We now allow taking screenshots when using --debug argument, and place the cmd line client directly in BINDIR. We maintain the same functionality for when using the keyboard.

Finally, we make use of weston_output_damage() such that the client will not be blocked waiting for the done event in the weston screenshot interface.