input-device: Only allow one renderer to consume input at a time

input-device: Only allow one renderer to consume input at a time

Right now if there are two graphics cards, there ends up with two
renderers active at the same time. Both process keyboard inputs
and both end up sending those events to plymouthd, resulting in
duplicate input.

This commit changes the input handlers so the first one wins, and
the rest don't get input.

Closes https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/197