Promote MultiplyEffect to a real effect.

The intended use is for overlays, where you'd want to do e.g.
0.2*x atop y instead of just x atop y, fading the overlay in or out.

Also, give it full RGBA inputs, as that might potentially be useful
for someone. It certainly was useful for adapting it to continue to
be used in the EffectChain unit test, at least. (It doesn't have
its own unit test, since it's so trivial.)
Properly link in multiply_effect.
A small comment fix in ColorspaceConversionEffect.