Initial flash of background before displaying plugin output

[mstoeckl]

When running swaylock --command 'program', the screen locker shows a background frame (white by default) until the background plugin program provides its first frame. Instead, Swaylock-plugin should wait (up to say, 200ms) for the background program to submit a frame.

[mstoeckl]

Update: now the screen locker briefly shows a black frame (rendered by Sway) because it generally takes a fraction of a second for the background plugin to render anything.