Estimate the error of an image by comparing two half buffers. If two statistically-independent images are each produced from half the camera samples, then we can estimate the mean-squared-error between the ground truth and their average and as one-quarter of the squared difference of the two. The AOVs may be either all scalar or all color. @parameters
aov1: Name of the first half-buffer AOV to read from.
aov2: Name of the second half-buffer AOV to read from.
result: Name of the AOV to write the estimated error to.