rework of MIP texture generation

[RandomBrushes]

This set of changes includes:

• MIP-textures now are restricted to colors already utilized in MIP0 (the original texture). This change is especially helpful for texture with fullbright parts, but also avoids using muddled in-between colors in other cases.
• Disabled dithering for MIP1 to MIP3. The introduced noise somewhat diminished the reason for having MIP-textures in the first place.
• If enabled, fullbright colors now take precedence over non-fullbright color duplicates (the Quake palette has some duplicates).
• Introducing the command-line parameter "-mip", with values 0, 1 and 2.
  1. 0 is high-quality downsampling
  2. 1 is averaging four pixel values, then selecting the original color closest to the average
  3. 2 is using high-quality downsampling, then selecting the original color closest to the downsampling result (default)

[lavenderdotpet]

if it doesnt already could this get added to the --help

[RandomBrushes]

Added a description of the MIP-algorithms to --help