Add a resize calculator

[Setsugennoao]

Like the one from AVSpMod

[LightArrowsEXE]

Should probably add a mode specifically designed for DVD cropping. Maybe make it a separate mode as well (like dft and descaling), where you can set the aspect ratio and try out different shifts, with a little balloon that explains what you should be looking for. I think that would drastically reduce the amount of poorly stretched DVD encodes around.

[Setsugennoao]

Better solution is https://github.com/Irrational-Encoding-Wizardry/vs-scale/issues/75