app: use gimp_transform_polygon() in gimp_transform_resize_boundary()
... 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.
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