Before you get too exceited -- no, this commit doesn't integrate
transform previews into the image graph :)  We still use a
separate canvas-item overlay, just like before, but instead of
using an impromptu implementation to render the preview, we use
gegl:transform.  We properly adjust the matrix passed to the op
according to the display scale, so that we still render only as
much as needed.

This is both notably faster than the current code, and, for
perspective transforms, more accurate.