... so that the transformed boundary is properly clipped.

Adjust the boundary-size algorithms to operate on arbitrary
polygons.

Avoid using gimp_matrix3_will_explode() in
gimp_drawable_transform_buffer_affine() and falling back to
cropping the result, and avoid setting the "clip-to-input" property
of gegl:transform.  Neither of those in needed anymore.

This effectively reverts the app/ part of commit
768d0661.  The next commit revets
the libgimpmath/ part.