Use more recent KTX-Software

[TimmRuppert]

The KTX Software submodule is 3 years old with the latest release of ktx software being from Mar 21, 2024.

For me personally an upgrade would make it simpler to use glTF-IBL-Sampler inside my own project as especially the KTX Software CMakeLists.txt got some major improvements:

• Fix CMARE_SOURCE_DIR vs. CMARE_CURRENT_SOURCE_DIR for includes and install
• cmake version requirments. As < 3.5 support will be removed from a future version of cmake.

If nothing speaks against upgrading KTX software, I could try to provide a PR.

[JMLX42]

Cf #33