Add descale helper

[Setsugennoao]

• [x] Add basic implementation
• [ ] Add GUI elements
  • [ ] Add min/max height
  • [ ] Add step mode/precision
    • [ ] Int/Float precision checkbox
    • [ ] Step increment
  • [ ] Graph resolution/size

[LightArrowsEXE]

Additional options may be:

• Crossconversion helper (every field upscaled separately, may have a vertical shift)
• Shifting (Zewia found some cases where the image was shifted after upscaling, and would show up as being undescaleable until the shift was accounted for)
• Allowing custom kernels, but that's probably not worth it considering the default set is already pretty solid for the most part
• ~~Removing cringe kernels or reworking how they're displayed (think spline and lanczos always resulting in relatively low error even when they're wrong)~~ This can be worked around by giving users the option to add/remove possible kernels for a getscaler-like functionality, or to have users define kernels themselves when selecting getnative (with some sane defaults to fall back to, like catrom).
• getscaler functionality, with a proper warning for newer users
• A help button that explains the concept of descaling very briefly

[LightArrowsEXE]

Additional ideas to make figuring out the kernel better:

• Have the descaling mode return three videonodes:
  • graph
  • src luma
  • descaled clip with current kernel
• Realtime swapping between kernels, videonodes update in realtime as well
• Swapping between kernels on a hotkey (ctrl+number, perhaps? Following the order of the kernels in the list of allowed kernels)
• Write the current kernel + any custom params on the clip using text.Text.
• Write relative error on the clip

[LightArrowsEXE]

Additional pitfalls that should probably have special cases applied:

crossconversion mode:

• Different vertical shifts per field
  • If the scaling was done correctly, every field should have a different shift (msg)