class function ClassNodeTypeName: string; override;
class function URNMatching(const URN: string): boolean; override;
function LoadSides: boolean;
Make sure all 6 sides are loaded. Make sure that all 6 fields for each cube map side are assigned, are TAbstractTexture2DNode instance (X3D spec requires it), load them, and check are they TCastleImage (we cannot suport TGPUCompressedImage when loading cubemaps this way, as we have to rotate images from ComposedCubeMapTextureNode when passing them to GPU, which is only possible for TCastleImage).
Also calculate AlphaChannel for the whole cube map. Our AlphaChannel method will reflect the state from last call of this.