It turns out GLTF is actually three different file formats. 😫 Textures can be loaded from external files, embedded in a binary .glb file, but they can also be saved in an ASCII GLTF files using base64 encoding. Having three different ways to store textures is not a good design decision, but at least it's better than the disaster called Collada. (Note to Khronos: If your file format specification has more pages than a Tom Clancy novel it probably sucks.)
Our GLTF loader now supports files with textures embedded in the file, in both binary (.glb) and embedded formats. We also support the Microsoft DDS extension, and support for the Microsoft LOD extension is on the way. The new editor will be able to save loaded models as GLTF files, and we can pack our own information away in our own file extensions, while keeping the file loadable by other applications.
Additionally, the new engine now supports textures loaded from DDS, PNG, JPG, BMP, PCX, PSD, GIF, ICO, TIF, EXR, and HDR formats, as well as Leadwerks TEX files. I'd like to get a new update out soon for the new engine, and then continue working on bug fixes for Leadwerks Game Engine 4.6.