Support the KTX2 texture file format

Qt currently supports KTX version 1. This version is now being replaced by version 2, which is significantly different: https://github.com/KhronosGroup/KTX-Specification/

The recommended tool for creating compressed textures, toktx, can now only create compressed textures in KTX2 format. The previous alternative, Mali Texture Compression Tool, is deprecated and will no longer be available for download.