Recently with the 18.0 release of our MBS Xojo Plugins, we added a new Clipper plugin part in the Tools plugins to use the Clipper library.
As you see, you can do polygon clipping with intersection, union, difference & exclusive-or using ClipperMBS module. The ClipperOffsetMBS class allows to add offset to a polygon like the one on top right.
And the bottom example is a rectangle with a triangle hole. We intersect with a second rectangle and get the new square area with the triangle hole.