Implement GammaExpansionEffect using ALU ops instead of texture lookups.

In a standalone benchmark (on a Sandy Bridge laptop), this is pretty much
a no-op performance-wise, but when more ops are put into the mix, it's
a ~20% FPS win, and but in a more real situation with multiple inputs
etc., it's subjectively also a pretty clear win. The reason is probably
that we generally are way overloaded on texture operations.

Note that we had similar code like this before (before we started using
the texture for lookup), but it used pow(), which is markedly slower than our
fourth-degree polynomial approximation.

We should probably do the same for GammaCompressionEffect.