Provide mip-mapping capabilities in libKTX

Currently, there doesn't seem to be a way to let the BasisU encoder generate mip maps. There is a host of options one can pass to the latest BasisU CLI:

  "Mipmap generation options:\n"
  " -mipmap: Generate mipmaps for each source image\n"
  " -mip_srgb: Convert image to linear before filtering, then back to sRGB\n"
  " -mip_linear: Keep image in linear light during mipmap filtering (i.e. do not convert to/from sRGB for filtering purposes)\n"
  " -mip_scale X: Set mipmap filter kernel's scale, lower=sharper, higher=more blurry, default is 1.0\n"
  " -mip_filter X: Set mipmap filter kernel, default is kaiser, filters: box, tent, bell, blackman, catmullrom, mitchell, etc.\n"
  " -mip_renorm: Renormalize normal map to unit length vectors after filtering\n"
  " -mip_clamp: Use clamp addressing on borders, instead of wrapping\n"
  " -mip_fast: Use faster mipmap generation (resample from previous mip, not always first/largest mip level). The default (as of 1/2021)\n"
  " -mip_slow: Always resample each mipmap level starting from the largest mipmap. Higher quality, but slower. Opposite of -mip_fast. Was the prior default before 1/2021.\n"
  " -mip_smallest X: Set smallest pixel dimension for generated mipmaps, default is 1 pixel\n"

Currently, while using libKTX works well for most other encoding-related tasks, all of the above has to be implemented separately (in our case using STBI resize, which sadly lacks Kaiser window filtering, IIRC).

Start by comparing the mipmap generation options listed in basisu_tool.cpp with the current libKTX encoding path. Review how the existing STBI resize-based workflow is used, then determine the scope needed to expose equivalent BasisU mip-mapping capabilities. Done means libKTX can provide the requested mipmap generation behavior without separate preprocessing.