A Winamp-style trail effect from two RenderTargets that
take turns (ping-pong). Every frame, a fullscreen pass reads the last
frame, zooms and twists it a little, blurs it and fades it into the
other target; then a fresh noise curve is drawn on top, and the result
is tone mapped to the canvas. The curve is brighter than 1, so the
float target keeps its glow. Click to reseed the noise.