Add n-wide side-by-side stereo images

[DylanSpeiser]

Allow stereo images to have a configurable number of horizontally stacked images. Generates n-number parallel stereograms. (Values around 4-6 may be easier to view than 2.)

[DylanSpeiser]

Hey I think this would be a good addition. Everything still works the same by default, plus there's added accessibility because the ones with four or six copies tend to be a bit easier to see than just two (at least for me).
A note: I tried adjusting the ranges and steps on the sliders for the number of pictures and the depth effect, but it didn't work on my machine for some reason.

[DylanSpeiser]

Fixed conflicts with gen_stereotb.

[semjonsona]

@DylanSpeiser Hello. Do you still want this functionality?

[DylanSpeiser]

Yeah I think it would be a good addition. I can fix the merge conflicts if you want.