Very clever! Thanks for the tip--I would never have thought to try "combine" on an image and a curve.
This is the same method, though: it avoids the distortion because the first 'combine' creates a curve that has the same size as the image, thus avoiding the distortion. You can see that the final result is actually a 'fill' if you look at the 'area' settings, but this is a
much easier way to get it done.
If it is important to you that OOo should have a true clip/mask feature, see
Issue 104035:
provide true image clip and mask by a shape
You can register there and add your vote (up to two) or comment.