2.1 alpha glitch
there is an issue with how alpha channel is processed with some FFGL plugins.for example I'm using a plugin called twisted or wave warp to affect the shape generator and the output image looks like a feedback effect that accumulates and never clears the previous frame. It won't clear unless the projector is deleted
the same here with ffgl panspinzoom or fr-parameters...
Please file bug report.
Have you tried to add or remove Premultiply?
This isn't handled correctly for all FFGL and may be related.