Cubemap support in Texture for custom materials and effects

Texture should enable creating not just 2D but also cubemap textures. (and eventually 3D (volume) textures but that's not part of this task)

This way custom materials and effects can expose and sample a cubemap in their custom shaders.

API-wise this raises some question, like what exactly the 'source' is then. A container format supporting cubemaps (ktx, dds?). Or also other formats, specifying the dimensions and position of the face images within the single image? Does it need a flag indicating we want a cubemap? Etc.

Screenshot from earlier discussion:

To clarify, this task is about the basic support which means supporting sourcing a CubeMapTexture from a container such as a .ktx file. Alternative sourcing methods, such as specifying 6 separate image files for the 6 faces can be added on top later on. Similarly, providing means to do procedural generation of face images (QQuick3DTextureData) is out of scope here for now.