libweston: Allow taking screenshots when debug protocol is enabled

Screenshots of the outputs can only be taken by having a keyboard
plug-ed in, as to avoid potential malicious intent. On the other hand,
this is problematic as there are cases where a keyboard cannot
be used as a input device. A particular use-case is that for
multiple devices it can be cumbersome to connect a keyboard such
that using ssh can be much easier and can be further automated.

This patch allows taking screenshots without the need of having a
keyboard connected when debug protocol is enabled.

Add also a few words about the fact that this is a serious issue
and can lead to silently leaking the output contents.

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