Effects need a preview toggle

[prokoudine]

Some of the effects are very slow, and all effects restart the rendering every time you change something. It would be lovely to have a preview toggle so that you can dial in your effects settings first and then enable preview.

[cameronwhite]

Were you thinking this would show a small thumbnail in the dialog where the preview is rendered on a small copy of the image? Or e.g. just rendering it live as usual, but only on a small portion of the canvas