fix upscaler 2 images do not match

[light-and-ray]

Description

I had a dream there can be a bug when the first upscaler rounds size to 8, and second upscaler doesn't because it's non-ai. But foutunatley it's okay, every upscaler rounds size to 8. (E.g. 1023 -> 1016) But due to this I've found another bug:

1. Set first upscaler AI
2. Set second upscaler lanczos

3.     return im1._new(core.blend(im1.im, im2.im, alpha))
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ValueError: images do not match
   

It happens because of different image modes

Also I've added minimum=0 and step=8 in mix_side_length, but it affects only on arrows next to number input

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