Basis/UASTC texture compression support via ktx2

[hrydgard]

Allows loading basis/UASTC textures in .ktx2 format, transcoding to whatever compressed texture formats are available.

To convert your textures to this format, use:

basisu texture.png -ktx2 -mipmap

or

basisu texture.png -ktx2 -uastc -mipmap

resulting in texture.ktx2. basisu.exe can be downloaded from here. The tradeoff here is that the first ones are smaller but worse quality and don't support alpha, while the second has full alpha support but is not quite as compatible with older hardware (I will add a fallback to decode to RGBA later though).

This allows for efficient and still cross-platform compatible compressed replacement textures - while not as fast as DDS to load, should be faster than png while still taking up a lot less memory when loaded. Will try to do some benchmarks later.

Currently causes some VK validation errors on the last mipmaps with rectangular textures, will fix before merge.