inverts the image held by image descriptor in and writes the result on the
image descriptor out. The function works on FMTUCHAR images only.
The result at point x of the input image is 255-x at the output image.
Input can have any no of channels.
This is not a generally useful program -- it is included as an example of a
very simple new-style IO function. Feel the power of the source, Luke!
The function returns 0 on success and -1 on error.