Removed LibKTX as a dependency.

Project now uses DTex for texture importing, added as submodule.
Added Vulkan-Headers as submdoule for future development.
Removed all internal TextureDocument structs. DTex will be favored.
Worked on getting compressed textures to work.

Signed-off-by: Nils Petter Skålerud <np_skalerud@hotmail.com>