Scale filter preserving alpha coverage

To properly support textures used for alpha-cutoff workflows, the mip scaler has to support an algorithm that preserves alpha coverage.

See http://the-witness.net/news/2010/09/computing-alpha-mipmaps/ for the implementation details.

Start by locating the mip scaler implementation in the KTX-Software repository and trace how texture alpha values are currently handled during scaling. Read the linked article for the alpha-coverage algorithm and determine how its behavior applies to alpha-cutoff workflows. Done means the mip scaler preserves alpha coverage for these textures.

Written by the indexing model from the issue text.