This cosmetic module replaces the grey levels of a Fimage with 
a fixed background level (\verb+bg+)
outside the maximal centered disk included in
the input image. A smooth (linear) transition (the width can be specified with
the \verb+-s+ option) is performed at the junction
between the image and the background.
You may want to call this module before calling \verb+frot+ or \verb+fftrot+.
\index{crop!an image}
\index{rotation!of an image}

