... which takes the symmetry axis as a parameter, instead of hard-
coding the axis to the middle of the image, and which additionally
takes the clipping mode as a parameter, controlling whether to clip
or resize the canvas.  Note that the actual canvas size never
changes, but it may be offset when flipped around an off-center
axis, without clipping.

Implement gimp_image_flip() in terms of gimp_image_flip_full().