According to the latest beta version:
WebGL shader effects can now use a 'scrollPos' vec2, which is the scroll position (in the middle of the screen) in layout co-ordinates.
This seems to mean that other than the following parameters (samplerBack, destStart, destEnd, seconds, pixelWidth, pixelHeight, layerScale, layerAngle, and viewOrigin) that are already exposed to your shader effects, you now have an additional parameter known as scrollPos.
This should be very useful for developing special shading effects!
Anyone tried it out yet?