app: use GEGL for transform-tools preview
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.
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