feat: additional --preview-interval values

[vmobilis]

Summary

Additional values for --preview-interval option of sd-cli:

• value 0 should preview only the last step of base-resolution sampling.
  Can be used in --hires generations to save (in --preview-path) the base-resolution result together with the high-resolution final image, skipping simultaneous high-resolution sampling + decoding.
  With feat: support numbering for preview images #1895, can save all base-resolution results of batch generations.
• values less than 0 should decode only the -Nth single image (to preview a sample and not to slow the generation down).

Additional Information

For example, --steps 20 --hires --preview-interval 20 and --steps 20 --hires --preview-interval 0 make identical "base + high" images, but with less memory.
Peak memory usage (SD 1.5, 256×512 -> 448×896, CPU):

--preview	--preview-interval 0	--preview-interval 20
tae	3.03 Gb	3.72 Gb
vae	3.12 Gb	4.33 Gb

Also, I have enabled "last step" preview for "high noise" video pass, but I didn't test any video generations and not sure if this is much useful.

The sd-server should be unaffected because it doesn't set the preview interval and uses default "every step" value.

Checklist

• I have read and confirmed this PR follows the contribution guidelines.

[vmobilis]

@leejet, could you please explain? If I uderstood correctly, in d9b6e27/src/runtime/preview_interval.h:

• the logical_sample_step(step) is abs(step);
• the sample_step_is_complete() is step > 0 || (terminal_sigma_is_zero && abs(step) == total_steps)
  (Because step == 0 ≠ total_steps > 0).

So the logic is same, except the case when step < 0.
When is it negative, I didn't find it?

Got it, I forgot samplers with 2 denoising passes, like "auto denoised_opt = model(x, sigmas[i], -(i + 1));".
Thank you very much!