mirror should work for image data

[jefferis]

Originally because I wanted a mirror_brain for images (see natverse/nat.templatebrains#18).

In this snippet from nat::mirror

    mirrormat = diag(4)
    mirrormat[mirrorAxis, 4] = mirrorAxisSize
    mirrormat[mirrorAxis, mirrorAxis] = -1
    if (is.null(warpfile) || transform == "flip") {
        xform(x, reg = mirrormat, ...)
    }
    else {
        xyzmatrix(x) = xformpoints(xyzmatrix(x), reg = mirrormat)
        xform(x, reg = warpfile, transformtype = transform, ...)
    }

If we converted mirrormat to a cmtk registration and then passed both of them to xform, then it should all work magically!

[jefferis]

Need to check behaviour when target image has non-zero origin since CMTK does not consider the origin of an image when registering or reformatting. See this question and answer on the CMTK forum. Essentially any image data will always be considered to have a non-zero origin and this goes for calculating a mirroring registration as well.

Therefore if the target space has a non-zero origin, points will have to be physically translated to account for this before and after applying the (image-based) mirroring registration. The same would apply for xform operations between different template spaces where the original origin would need be removed and the new one added. This is not a very satisfactory state of affairs since the assumption is that a bridging registration should be identical regardless of the kind of data being transformed (though in practice we already need to select either a forward or inverse direction depending on whether we are dealing with points or image data).