Add support in Ktx textures for FXT1 compressed textures.

PxFmt library already supports this format but Ktx textures didn't.

Existing ktx texture code assumed compression blocks were square texels. Since
FXT1 compression is a non-square block (8x4), this required additional changes
to support non-square blocks.
Remove mipmapped_texture class and files.

Removed last usage of mipmapped_texture class.  Most straightforward way
was to just remove the last unused function (read_image_from_memory()) that was
using mipmapped_texture class.