add style=flash to spacescope.

the goal was a 1 frame of white whenever a "audiotestsrc wave=ticks" tone was detected.

I am seeing 46.88fps on a 30 fps stream, so I think many of the white frames are getting lost.

I can make this work by playing with the new audiotestsrc parameters, but maybe this blipbop generator should be written from scratch as a single plugin that generates both.