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 (not just TEncodedImage; loading S3TC cube map sides to OpenGL is not available).
Also calculate AlphaChannel for the whole cube map. Our AlphaChannel method will reflect the state from last call of this.