RGBA -> RGB when no transparent pixels, cleanups

- RGBA/DXT5 -> RGB/DXT1 during mipmap generation when no trapaarent pixels
- `*/FX/*` and `*/Spaces/*` added to mipmaping-allowed paths
- refactoring, more comments
Custom kerbal heads

- TextureReplacer
  * custom kerbals heads support
  * textures can reside anywhere in `GameData/TextureReplacer` except in special
    `CustomKerbals/` dir
  * `*/FX/*` and `*/Spaces/*` added to mipmap generation
  * when generating mipmaps RGBA & DXT5 are checked if opaque and converted to
    DXT1 if neccessary