[Bug] Fix extras big batch crashes

[light-and-ray]

I've found postprocessing in extras tab stably crashes for big batches. It happens because
shared.state.assign_current_image(image_data)
And later:
image_data.close()
Inside postprocessing.py

And also I think it's better to show processed image in preview instead of input image. I decided to fix this crash with using pp.image instead of image_data for preview

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests