Mipmap bias

Added suppport for skipping first N mipmaps (including original image), so this effectively works as a texture reduction. Number of skipped mipmaps can be configured in a `.cfg` file which contents looks like:

    DDSLoader
    {
      mipmapBias = 1
      normalMipmapBias = 2
    }

`normalMipmapBias` specifies bias for normal maps, while `mipmapBias` for other mipmapped textures. Both values shoud be 0 in the default configuration file, of course.

Additionally, BGR -> RGB conversion code has also been simplified since you don't need to worry about alignment here in Unity.