Fix/skip img2img

[hananbeer]

In settings there's a skip_img2img option but it does not work at all.

This patch makes some small changes:

• Skip img2img when the setting is enabled
  • Also skip img2img if preprocessor selected is "none"
• Skip loading model if "none" is selected (it is not needed when skipping img2img)

Note that regardless of skip img2img you must select some image in the main input image field. That is because the first thing it does is call img2img before any callbacks which tries to convert the image. So be aware of that.